Such treatment can take the form of being criticized for a shortcoming that was previously overlooked, such as being occasionally a little late for class.

Most definitely. Reminds me of an experience where I refused to put in an appearance at a kindergarten for no PAY, unrelated to the public school I was working at, as requested by one of the fellow chinese co teachers. If you have a mobile number, don't give it out to chinese co workers, better yet, don't have a phone so they can't reach you. The result of my refusal to help them claw their way up the guanxi ladder (which only goes their way, not mine) was any extension of my contract was refused. An american teacher, who also disliked some of the chinese teachers attitudes/racism and was outspoken about it, also was refused to stay on.

Are the Chinese vindictive? My experience tells me some can be, but most are too laid-back to be so!

Don't be taken in by the smile and supposed friendliness. If they hate you, the smile is just a mask. Most of the time you won't even know face is on the table, let alone whose face you are offending and what face it is they have got on. They can be vindictive, in simply not renewing your contract for whatever childish reason, or refusing to give you a release letter.