The role of southern communists during the war was actually somewhat muted compared to Vietnam. There were still active guerrilla fighters, especially in the mountainous regions, but circumstances pre-war and the way the war played out in 1950 goes some way to explaining exactly why they lost significance compared to their pre-war activities.

Korea was put in a rather strange position in 1945, in that political views were well-dispersed across the country. For example, South Pyeongan province, and Pyongyang in particular, were heavily Christian whilst the two Hamgyong provinces were hotbeds of communist sentiment, mainly due to the mountainous geography providing a safe haven for the communist guerrillas who had been fighting against the Japanese occupation. The communists actually had a decent following in the south, under the nominal leadership of Pak Hon-yong.

When the US Army Military Government in Korea (USAMGIK) set itself firmly against the communists, suppressing the (not necessarily communist) local people's committees that had sprung up and instead working with right-wing nationalists like Syngman Rhee, the more militant communists saw the writing on the wall and either fled to the north or took to the mountains. There were three main areas which saw particularly strong communist activity - the Taebaek and Jiri mountain ranges, and the island of Jeju-do. In the years between liberation and the outbreak of war, there were some major uprisings which caused significant concern to the USAMGIK and the fledgling South Korean government. Now granted, these weren't solely communist affairs, but the communists did effectively take leadership of these activities.

In autumn 1946, an uprising occurred which spread across much of the southern zone, primarily caused by USAMGIK's decision to remove price caps on rice which led to speculation and hoarding, leaving many without sufficient food and harkened back to the days when the Japanese would requisition Korean rice for their Home Islands, whilst according to one account I've read supposedly 'inferior' rice was imported from Indochina for Korean consumption. This, in concert with the general retention of colonial-era officers and administrators by USAMGIK caused substantial resentment. The rising was put down, and the Korean National Police (KNP) was strengthened whilst a reserve Constabulary was also set up to help against such affairs. The KNP was very right-wing, but the Constabulary, which would later become the ROK Army, was more lenient on its membership which would be important in 1948.

1948 saw a much stronger, and more strongly communist, uprising on Jeju-do. The guerrillas on the island were based around the central Halla mountain, and sympathisers in the Constabulary units sent to quash them then rebelled in Yeosu which spread the insurrection to the mainland. This activity took some time to put down in the main, and those who managed to avoid the now-reformed ROK Army took to the Jiri mountains and holed up around Halla mountain for months. The ROK Army eventually managed to clear up most (but not all) of these, making it an effective counterinsurgency force by 1949. Note that the south was very strict against rebels, and whilst there's a lot of public Western knowledge of the atrocities committed by the north, Rhee's government committed its fair share of mass executions.

Two more insurrections happened around 1948-49, both much smaller, in Daegu and near Pohang. Both involved ROK Army units and both were put down, with the units involved being purged.

Meanwhile, the Taebaek mountains were a hotbed of activity mainly due to their proximity to the 38th Parallel, which gave the north opportunities to filter their agents in to coordinate guerrilla activity. The border at this time was nothing like it is today - there was still great hope of a reunified Korea so it was much easier to cross, even though it had supposedly been closed. Several accounts of those who fled the north did so by hiring guides to take them through concealed routes. Again, by the outbreak of war this group of rebels hadn't been fully crushed, but it had been weakened.

Now for the war itself. These groups sprang back into action with the north's invasion, and they joined up with KPA troops as they pushed southwards. As the UN forces started to push northwards again, many of the guerrillas had by now come out of hiding and were caught up in combat. The nature of the guerrillas changed as dedicated KPA regulars, outmanoeuvred by the speed of the UN advance, now took up the fight instead and continued fighting from the mountain hideaways. Many other communist sympathisers instead took the opportunity to flee to the north, fearing the bloody reprisals that Syngman Rhee's government would exact upon them.

Ultimately, during the war itself the guerrillas failed to make any major impact. They were a nuisance which tied up men and resources, but for the most part they were effectively contained to their respective areas. It's partly due to the geographic differences between Korea and Vietnam - in Korea, you have mainly mountains or plains, the latter being more common in the south, whilst in Vietnam you have dense jungle. Furthermore, the Viet Cong had a major advantage in support through the long borders with Laos and Cambodia, through which the Ho Chi Minh Trail kept them supplied with men and materiel. South Korea's only land border was with the north, so supplying the insurgents was much more difficult, relying instead on whatever they could scrounge from their surrounding areas (either voluntarily or at gunpoint).

That's the military aspect of it. Pre-war the communists were also active in other ways, like forging banknotes and the like. Again, this had been largely suppressed by the war, and again many of those involved fled in the face of the advancing UN forces in late 1950.

Further Reading:
Miyoshi Jager, S., Brothers at War (New York NY, 2014)
Kim, S., Everyday Life in the North Korean Revolution, 1945-1950 (Ithaca and London, 2016) - Mainly for chapter 7, which draws on some accounts of the communist experience in the south.
Korea Institute of Military History, The Korean War, vol. i (Lincoln and London, 2000)
Mahoney, K., Formidable Enemies (Novato CA, 2001)
Millett, A.R., The War for Korea, vol. i (Lawrence KS, 2005)