We should be careful about loosely tossing around the word "deportation." True deportation from any country is a legal process with a lot of paperwork and officials involved. I don't know about Vietnam, but China generally reserves the actual deportation process for a select few foreigners who have egregiously flouted laws, usually criminal laws. A blacklist from future access to China for a time period or forever, is also part of the deal. For minor infractions like not registering (if the unregistered period is relatively short), they tend to issue warning and fines. For violations that like somewhere in the middle like most civil transgressions (including visa overstays of a few days to weeks), they usually just unceremoniously cancel, revoke, or alter the terms of the visa to allow a short period (typically 3-5 days) to pack up and exit the country + levy a fine. Being shut out of China for a few years is frequently part of this deal also, but it's far short of true deportation. Usually a deportation edict follows one around for a long time, as most other countries using electronic passport processing can see it. The more informal Chinese booting-out may or may not leave a trail that other countries can see.

It's best to avoid being on the wrong side of Chinese law/regulations, whether it's being "just a little bit wrong" or "really far out there" crossing over the legal line.