Flashman,
There’s an event card that implies increased Chinese assistance if the US player does certain things, but no actual Chinese units or direct intervention (well, there are some Chinese freighters that are delivering supplies in the game, but no combat units). The game doesn’t explore any possibilities of any US incursion into North Vietnam (something that would never have been sanctioned by any President), although you can get too aggressive and risk losing US “National Will” in all sorts of ways (invading Cambodia, for example).
As to why Vietnam and not Korea, Korea may be the next game we do! Vietnam is simply the designers singular passion, and he is a true expert on the subject, so that’s how this game came into being.
:-)