Let's assume no direct Chinese or Soviet intervention.
What are we talking about in terms of an invasion force? What's available? A sortie of 100,000? 200,000? 500,000? Believe it or not, there's a limit on the number of soldiers available for the meat grinder. We have major commitments in Europe, in the Phillipines, in South Korea, Japan, South Vietnam, Thailand, etc., troops at home, etc.
When does this invasion take place?
How much is it going to cost?
In South Vietnam, we placed up to 500,000 troops with logistic support in a friendly government. The government we're invading will not be friendly, will not be providing or supporting logistics. Everything we move in and build will be under fire.
This would be a scale of undertaking equivalent to Inchon or Normandy. Not something easily undertaken. Do we still have the amphibious capacity? Or are we going up the coast.
We can anticipate incredibly heavy fighting every step of the way, before we wipe out conventional military forces. Urban fighting especially is going to be a bloodbath.
After that, the North Vietnamese communists melt into the population, and we're getting ambushed every time we turn around. Good luck winning hearts and minds after we napalmed half the country getting in there.
The Ho Chi Minh trail will start at the Chinese Border, almost no way to interdict it without buying a serious throw down with China.
No happy resolution. Five years, ten years later, we're still in Vietnam, the body count is huge, the SVA government is even more of a joke, and we can't get out.