You're misinterpreting it, I think. IOTL Tet destroyed the VC and the NVA lost conventionally as well when they attacked. After that the VC never regained its strength even as Americans drew down their forces.
We're not changing what happens there—all we're changing is the political impact on the American public.
So. Everything in Viet Nam that happened in real life continues to happen up to and including Tet, but the political impact is changed in America.
Your point is relevant to an alternate Tet with a smarter North Vietnam, but we're accepting the OTL Tet where the North Vietnamese were stupid (militarily speaking) and lost badly.
Yeah, that's the point I was trying to make. Thanks, Monk! Anyway, would the war have changed at all if Tet was not as much of a political victory for the communists as it was in OTL? Some discussions in this thread seem to point that it wouldn't have, and that the war would have kept plodding on until the US just got frustrated with it all (which I suppose is what happened in OTL anyway).
Bringing up a point noted earlier, I think one of the most important factors in a post-US-"won"-Tet Vietnam is the reaction of China. I'm not one for knowing the stability or effectiveness of China circa the Vietnam War, but how nosy could they have gotten? Furthermore, would the NV and VC even allow China to operate, given historic anti-Chinese sentiment? I'm partial to thinking that the Vietnamese wouldn't allow it, but it'd give rise to an interesting TL nonetheless.