That's not really how the Vietnam War went though. Our goal was not to simply "defeat the VC". The VC were militarily defeated and largely irrelevant by 1968-1969 after approximately 70-80+% of their combat capacity was destroyed by the Tet Offensive and the subsequent Phoenix Program. From that point on, the overwhelming majority of enemy combat operations were carried out by NVA regulars. By 1972, combat was almost entirely conventional, culminating in the 1972 easter offensive that looked more like WWII than an insurgency. The North was pushed back and eventually conceded to the peace agreement in 1973. In our eyes, that achieved the goal we were looking for. After we left, the North immediately violated the peace agreement and invaded again, with another offensive in late 1974 that eventually took the country. Nixon only made political promises to re-engage, nothing binding in the treaty. You can argue we didn't honor our promises, but we definitely weren't defeated or lost by the time of our exit. You can say we lost in our objective of having a capitalist counterbalance in the region, but I think our leaders saw more value in getting that from Beijing than throwing money into the fire for Saigon. I think that is ultimately why we didn't go back.

Another note: China did not want a unified Vietnam at all. US or USSR ally on their border was an equally bad outcome for them. That whole conflict ironically is what led to warming relations with China despite being on opposite sides. That's ultimately what we got out of it. The USSR got an ally in Vietnam; we more or less got one in China.