Well you can look at the occupation of Vietnam between 1407 and 1416 (after that it was in war time so it counts less). There was a heavy sinisation campaign with ban of tattoos, more confucianism, re-introduction of chinese, Chinese clothes...How much cultural integration is really necessary? Take Guangxi, for example. One could argue that independent Vietnam was as much or more a part of the Chinese cultural continuum. I suppose it's probably not too late for the place to become a province indefinitely, depending on how those overthrowing the Yuan choose to handle things.
To me that means that there was quite a cultural gap, otherwise they wouldn't have needed it