The image of the average guerrilla soldier of the People's Army Liberation Force (PLAF, known pejoratively as Viet Cong in South Vietnam) as a poor rice farmer, holding an antiquated rifle, dressed in all black garb topped off with the conical nón lá hat on their head was one that was established during the war itself. It was an image that the National Liberation Front (NLF) and the North Vietnamese government (DRV) wanted to cultivate in order to gain support. The North Vietnamese government quickly recognized how absolutely vital international opinion and support would be in order to win the war. What if you managed to appear as the only authentic voice of a unified Vietnamese people? What if it was your brand of nationalism that could only be considered authentic while you demonized your enemy as a puppet, as something non-Vietnamese? It is to the credit of North Vietnamese and NLF propaganda that we still think of a PLAF soldier as the rag-tag farmer of popular culture. Yet this exists outside of popular culture as well. For a long time in the Vietnam War historiography, many continued to follow this understanding of the war due to an overly America-centric scholarship. But let's go back to the Vietnam War before we get too ahead of ourselves.

Is there any actual truth to this image? Yes, there is, but not in the way it is portrayed in popular culture or war-time propaganda.

The PLAF was divided into three different categories of soldiers and units which all served their own specific role. At the bottom, you have your classic part-time guerrillas: The local forces. These were men and women who worked within a specific area, often their immediate hamlets, with specific tasks that did not often involve actively fighting against 'Free World' forces. This could be anything from building hidden caches to reporting intelligence. These soldiers would be equipped with whatever they could get their hands on, reflecting their often limited state of training. Above that you had the regional forces. As the name implied, these soldiers worked within larger geographical areas (but still limited) and were also part-time guerrilla soldiers. Yet they would often serve in support to the full-time guerrilla soldiers: The Main Force PLAF. The main force (often capitalized as Main Force) units consisted of full-time soldiers who were trained and organized as conventional soldiers. The Main Force units would be equipped with the latest weapons and be dressed in uniforms, with a boonie hat as headgear (and not the nón lá that none of these three types of soldiers would wear in combat). Additionally, these units would cross into different provinces away from their homes unlike local and regional forces. It was the Main Force units that most commonly engaged 'Free World' forces in active combat.

We therefore have different types of soldiers who served a specific purpose. While they all fall under the PLAF umbrella, they are not the same. Some partly did fall under the notion of a part-time farmer soldier of popular culture, but only in some specific ways. The real historical origin of this image lies in North Vietnamese propaganda.

To understand the National Liberation Front's strength in the South, one has to go back to Operation Passage to Freedom, where large numbers of Viet Minh cadre were ordered to stay behind and form the nucleus of communist operations in the South. An estimated 5,000 to 10,000 cadre stayed, not including others in the Viet Minh apparatus and organization.

Now initially, what these cadre did was not organize Southerners into combat units or even guerrilla fighters, but instead focused on recruitment, intelligence, and propaganda, as well as constructing forward operating bases and other infrastructure—laying down the foundations of future struggle. They were quite successful in recruiting from a largely alienated rural Southerners, the peasant farmers. And so at this point, you could genuinely have called the "Viet Cong" a bunch of farmers. The NLF did not really start violent struggle until 1957 or 1958 when it began assassinating South Vietnamese officials wholesale. They did so because of the adoption of an aggressive policy towards the South after a reshuffling of those in power in the North.

However, even though they had not really begun open fighting until 1958, the NLF assassination program of Southern officials was quite robust leading up to that. While numbers vary quite a bit, Bernard Fall estimated that the South had lost almost 20% of its village chiefs through 1958, and that by the end of 1959, they were becoming casualties at the rate of more than 2% per month. By 1960, the South was losing 250 to 300 officials a month to targeted killings and abductions.

Of note here, the National Liberation Front at its source was almost entirely directed by Northerners. While early on, it was composed of Southerners, they were in constant contact with the North and received supplies to include weaponry from the North and orders as well.

And so, the "farmers" being trained by veteran Viet Minh cadre were shaped into an efficient guerrilla force, at first, armed with vintage Japanese and captured French firearms, in addition to their home-brewed guns. Defectors from the ARVN also brought their US-sourced firearms, and the North would ship communist-bloc sourced firearms as well. And so early on, they would have been armed with bolt-action rifles and submachine guns, they increasingly became armed with more and more modern weaponry, not to mention their heavy use of light machine guns and general-purpose machine guns. In many instances, they would have entirely outgunned (in terms of small arms) the ARVN they came up against. Additionally, the proliferation of the RPG-2 gave these "farmers" an effective anti-armor tool. By 1967, they had largely been armed with the AK pattern of rifle, which absolutely blew anything the ARVN had except for the "elite units," the Airborne Division and some Ranger battalions, who had been armed with the M16 rifle. Not until 1968 and later did regular ARVN units become equipped with M16 rifles and M60 GPMGs, and so at least for a year or two, the ARVN was largely using WWII vintage M2 carbines, M1 Garands, and BARs against the cutting edge of small arms technology—a situation that heavily demoralized Southern soldiers.

Now by the time open conflict broke out, it would be hard to even still call these individuals farmers. the NLF was really broken into two main groups, the political/propaganda side of things, and then the combat side of things. Unfortunately, the vast majority of pop history on the Vietnam War does not distinguish between the two at all, and neglects to discuss the fighting branch of the Viet Cong in detail. The fighting side of the NLF by mid-war, was organized into what was called "main force units." These NLF soldiers were organized often on the battalion or regimental level (and even sometimes on the divisional level) and served the same role as regular PAVN units. They were no longer peasant farmers conducting sporadic guerrilla attacks, but organized, armed, and equipped as conventional soldiers.

The 1968 Tet Offensive and subsequent "mini Tet's" devastated the National Liberation Front as a fighting force. Between 40,000 and 75,000 NLF soldiers were killed or became casualties between 1968 and 1969. And so after Tet, the North sent over 140,000 Northerners down to bolster the ranks. NLF main force units became increasingly composed of Northerners after Tet, as they just could no longer recruit as effectively—the controversial Phoenix Program, which conducted targeted assassinations of communist cadre, while entirely dubious in its overall effectiveness, was effective in driving communist recruitment and organizational efforts far deeper underground than they had previously been.

I think most notably, is that once final victory was achieved in 1975, the North dissolved the southern communist government and removed many of its officials from power, replacing them with Northerners—the Provisional Revolutionary Government had never been more than a tool for the North and the North treated it and its Southerners as so.

For further reading on the origins of the National Liberation Front, I highly recommend Volume 1, Chapter 5 of the Gravel edition of the Pentagon Papers, the section on "Origins of the Insurgency in South Vietnam, 1954-1960."

This comment by /u/quangtit01 is somewhat related to your question, specifically this section:

+/ The French Imperialists and Japanese Occupiers of Vietnam between 1944-1945.

+/ The first Indochina war between 1945-1954 against US-supported France, resulting the French's defeat (culminating in the famous Battle of Dien Bien Phu)

+/ The Vietnam War between 1954-1975 against the US-supported South Vietnamese (1), resulting in the annexation of South Vietnam

This doesn't directly answer your question, but Vietnamese troops thus had a lot of experience due to decades of guerilla warfare. However, your question is not about the Vietnamese Army, which was battle hardened and professional, but the Viet Cong.

It is correct to characterize the Viet Cong as significantly less professional than the Vietnamese Army. They didn't win their first combat encounter against the South Vietnamese for several years after becoming active, and were never a significant military threat by themselves. However, the Viet Cong were extremely useful in two ways:

1. They provided support for Vietnamese Army maneuvers in the South

2. Their presence across the country-side gave the Vietnamese Army military flexibility to act in many different places at once, allowing them to maintain the initiative across the region.

As such, maintaining this force was a priority for the quite capable North Vietnamese military planners, and they extended a lot of effort to make sure that the Viet Cong was as best prepared as it could be to support the Vietnamese Army in its mission.

The Viet Cong's training and equipment was variable. There are two main groups you should consider - the full-time professional soldiers of the Chu Luc, and the paramilitary militias, mainly southern, aligned with them in the National Front. The Chu Luc were comparably well armed to the South Vietnamese forces, in terms of small arms. The Vietnamese Army, through Group 559 with prioritized establishing logistical networks, had established the Ho Chi Minh trail and later the Sihanouk trail as a very reliable supply route to arm local Viet Cong groups, and there was no shortage of Soviet weaponry available. In general, the Viet Cong was given weapons the Vietnamese Army no longer needed - when the Vietnamese Army started receiving automatic rifles, such as the AK47, they gave their old submachine guns and carbines to the Viet Cong. In this sense, they were somewhat inferior, but by the mid-point of the war there was such a surplus of equipment that it wouldn't be unusual to see professional Viet Cong units deploying anti aircraft batteries and heavy machine guns to support their infantry maneuvers and wielding identical weapons as the North Vietnamese Army itself. In fact, the Vietnamese Army was so overwhelmingly successful in establishing secure logistical networks, that by the time of the Tet Offensive, the characterization of the Viet Cong as a rag tag and poorly armed group of farmers was only even somewhat true in the most secure and defended of the South Vietnamese provinces where North Vietnamese logistical networks could not penetrate to deploy their own troops and equipment.

As for your second question - yes. The 1968 Tet Offensive was a major Viet Cong victory, but it came at a heavy cost to manpower, especially to local units in deep South Vietnamese territory. By the end, the Viet Cong had been decimated, the southern units especially, and by 1969, estimates said that more than 3/4 of Vietnamese units operating in South Vietnam were composed of northerners that had come south to volunteer.