Before we go into the finer details about General Giap's generalship and why he is considered such an effective general in the field, there is one thing we have to make very clear:

Guerrilla warfare and counterinsurgency is not won by winning battles. For the guerrilla, it's all about surviving and working both the political as well as keeping up military efforts to make the people know that you are out there, that you are not to be defeated and that you will outlast your opponent. In counterinsurgency, it's about countering this view through socio-economic and military means. The less shots fired and the more guerrillas who surrender without a shot - the better.

By seeing asymmetrical warfare out of the perception of conventional warfare, you are doing the same mistake that lead conventional armed forces into these sort of conflicts without any preparation or institution to adapt. It is this obsession with conventional warfare that led to the United States losing.

Anyhow, with this in mind, I will now speak about General Giap and the focus will not be on the American War (i.e. 'The Vietnam War') but rather on the far more, in this context, important French Indochina War.

What Vo Nguyen Giap did during the fateful years of 1946 and 1954 was to take a rather new and in the field proven guerrilla warfare theory which had yet to lead Mao Zedong to victory (he would in 1949) and adapt it to Vietnamese circumstances. In the Chinese example, the war was supposed to progress in stages: A prepatory stage which was to make the civilian, the peasant, aware of the struggle and make them join your camp - a guerrilla phase which was all about building up a base and going on the offensive using assymetrical means while you keep building that base and build your army - a conventional phase when you surrender the assymetrical means for the conventional means and go onto a conventional offensive.

Giap took this theory and made it his own. There was a prepatory stage and guerrilla phase between 1946 and 1949, mostly due to the fact that an insurgent group really can't survive on the little supplies that were available and without any proper outside help since the Viet Minh was essentially stuck in the Viet Bac, the border area between Vietnam and China. However, in 1950, things came around. With an influx of weapons, supplies and training from the Chinese, he finally found a basis on which to defeat the French on. Giap began to play around with the theory and used it to essentially blend stage 1 and stage 2 together into one with just a dash of stage 3. Using highly mobile units, Giap took advantage of every little weakness the French had: A lack of men? Stretch them out from the coast of Vietnam all the way to Laos. The French expeditionary army being highly mechanized and kept to roads? Ambush them. Ambush them constantly until they can't take it anymore. The French keep static bases in the border region? Lay siege to one of them, and the French will try to save them. When they do, ambush them all the way there and all the way back.

And this continued. He infiltrated into the Red River Delta. He bounced back after a disastrous attempt at a conventional offensive in the winter of 50/51 but when the French captured Hoa Binh from the Viet Minh - he turned the table on them and managed to encircle them. These things are obviously nothing in comparison to the battle of Dien Bien Phu which is truly Giap's masterpiece, his Cannae. While the decision to fight at Dien Bien Phu has been discussed amongst historians for decades and while new scholarship has pointed out Chinese influence in the decision to strike there, the actual plan of what was going to go down was all Giap. /u/InTheCrosshairs has already mentioend what happened there but there's one thing I really want to emphasize: This was a guerrilla army becoming a conventional army. At Dien Bien Phu, the French Army was beaten at its own game. At Dien Bien Phu, Giap not only reached stage 3 but also finished the war. It is simply a stroke of military genius and should be appreciated as such.

And this is just one war. While General Vo Nguyen Giap did not really had as much control in the field during the American War (but served more as the Head of Chief of Staff and overall strategy), there were plenty of other very fine North Vietnamese generals. However, your characterization of People's Army of Vietnam after the Tét offensive is wrong. The PAVN managed to continue go on offensives afterwards, using mobile groups to do several 'mini Tét' in the months that followed.

While I wouldn't say that the NVA "won" any battles outright, I also wouldn't say that they were completely annihilated either. They accomplished several of their goals at Tet, and most definitely accomplished their goals in their insurgency efforts. While this does not an entire victory make, it's important to put into context that the United States and the Vietnamese were employing two very different strategies. At Tet, both sides claimed victory, because the two sides were using two completely different measurements of success. It seems as if you're holding the American Vietnam War strategy of "War of Attrition" up to Giap and not seeing things line up, which is understandable.

When you consider that Vietnam had only truly been a free country since the aftermath of the French Indochina War, it absolutely does make sense that it would be hard to keep an army together. The Chinese only broke through to the Viet Minh in 1949, just 8 years after the communist rebel groups unified under the Viet Minh leadership in 1941. In the grand scheme of things, this all happened very, very quickly. When you also factor in that the communist rebels had more or less been fighting since WWII, not only in WWII but in the French-Indochina War, it begins to seem a more daunting task to field an army that has been fighting on and off for 30 years.

As for a maneuver or exploit of his, i'll stick to the two most famous ones: Dien Bien Phu and the aftermath of Ia Drang. At Dien Bien Phu, he lured the French into Laos, and set up a trap consisting of well-placed anti-aircraft guns and trenches that trapped the French in a valley that was nearly impossible to resupply. The Vietnamese surrounded the French and in March of 1954 began a slow, steady tightening of the noose that lasted 54 days and ultimately caused the suicide of the French artillery commander when he saw how beaten they were. It was a huge victory for the Vietnamese forces and the next day the French government announced it's intention to withdraw from Vietnam, and followed up on their promise. At Ia Drang, Giap showed his flexibility by observing the American's tactics and changing the entire way the NVA fought the Vietnam War. Before Ia Drang he attempted to fight conventionally, after his defeat he changed his strategy to the insurgency strategy that the Vietnamese are infamous for today. This accurate reading of the Americans in a situation where he was more or less assured defeat based on the outcome of Ia Drang is nothing short of a miraculous turnaround for an army that was crushed. While the Americans won the battle of Ia Drang, Giap's strategy that came of it prolonged the war and allowed the Vietnamese to gain a form of victory in 1975.

I understand the diplomatic aspect of getting the Western press worried about US victory, but is this really a military achievement?

/u/Bernardito has given a better summary of Giap than I could hope to deliver. However, I can answer this. In short, yes, that is a military achievement. Warfare is not a game conducted to see who can achieve a better kill-death ratio, war is about achieving a specific goal through violence. That violence does not have to take the form of actually winning pitched battles or acquiring territory - just as important as either of those is damaging the morale of the enemy, both their military forces and that of their home front. This is also an important thing for the opposite side of a guerrilla war, which is why you will hear about counterinsurgency theorists talk about such things as "hearts and minds." The US won practically every engagement against the NVA and the VC, but by prolonging the war and inflicting consistent (if not that damaging from the tactical perspective) losses on US forces that turned the US public against the war, the North Vietnamese were able to drive the US out. Considering that the US basically withdrew from a conflict in which it had achieved several victories and had an overwhelming technological and tactical advantage, that is a pretty significant achievement on the part of the Vietnamese. Defeating your enemy by ruining their will to fight is no less of an achievement as defeating your enemy through grand maneuvers.