As was said above, it doesn't matter where you're flying in from.
The official answer on using it for personal travel is "maybe". It's in the FAQ at
https://www.apec.org/Groups/Committe...Group/ABTC/FAQ with the answer
The ABTC is not designed for tourism purposes, but some economies may facilitate entry for cardholders depending on their circumstances. Please refer to the relevant economy for individual entry arrangements for cardholders and tourist visits.
Simply using it to get access to the faster line at immigration will not be a problem (worst case they punt you back to the normal line, but I can't see that happening). The issue would be countries where you need a visa - which includes Vietnam. If they decided to enforce the "business travel only" use, then at best you're going to have a potentially difficult time getting through immigration. I have seen at least one report of someone being refused entry as they were on non-business travel, but I don't recall where that was mentioned (might have been FT) or what country - and of course you'd likely never know the full story/reason...
Other than countries where there's a option on the immigration form (eg, Thailand) I don't actually recall ever being asked if I was there for business or pleasure when using my ABTC.