If China did join they would have to allow independent trade unions among other things, which is a big ask. Vietnam caved and allowed it because they were so desperate (as the poorest country in the TPP they have a lot to gain), but China will have a lot more bargaining power. Of course, the other problem will be that a Free Trade Agreement with China (and other possible entrants like Indonesia and India) will be enormously controversial domestically.
I don't believe the Vietnamese communists have any genuine desire to comply with the independent trade unions rule. They'll simply create a facade of independence with the official trade union while maintaining strong patronage links between the trade union and the party. Because they'll maintain plausible deniability, the US will simply pay lip service and move on, as it usually does when its friends break rules it sets.
