Hold up there my nigga.

fftopic: Tet misinformation a personal pet peeve of mine.
Tet was a complete fucking disaster for the North Vietnamese in everyway except for PR. They were unable to hold any of their gains, within weeks they'd lost anything they'd gained, and lost hugely disproportionate men and material in the assault and subsequent counter-attack. Militarily, they were in really bad shape after Tet. The NV generals also burned through a lot of their hardened VC partisans by trying to use them like normal infantry with bad results once the surprise wore off.
South Vietnamese troops, by contrast, were showing that almost a decade of "advising" was finally starting to pay off and they were able to fight the northern troops in straight engagements. Had congress approved the additional troops & equipment, it is likely North Vietnam would have been unable to do much to stem a counter invasion, and probably would have been pushed to the point they'd have had to accept a Korea-like situation.
(In all likelyhood they could have been overrun and defeated with requested troops and material, but the Soviets wouldn't have stood for that; the Chinese reaction (which is what turned Korea from a UN victory to a truce) is harder to extrapolate: there key differences in Vietnam as they were on the outs with Soviets at the time, and didn't feel a "kinship" with the Vietnamese like they did the Koreans. The Chinese government also had secured its grip on the country; in 1950 they feared that the UN wasn't going to stop and keep going into China and possibly take Manchuria away, nearly 20 years later, in a better place internationally and internally, they might not have felt a need to respond.)
Westmoreland was completely right tactically, and if he'd gotten what he wanted, Vietnam would have been a victory. The optics were terrible; instead of a request for a conquering army to finish a weakened foe, it looked like a panicked request for troops to stem a retreat (especially since he'd wanted them as quickly as possible, to be able to hit before the North could be resupplied by the Soviets).
But, then the question to ask is: What sort of victory? What would 50,000 more american lives and who knows how many wounded have gotten us in the long run? Whatever hundred of millions of dollars... what would America get for that?
South Vietnam's government was pretty shit. The corrupt politicians wouldn't call themselves communists, and that would be about that.