The Tech scene in Singapore, to answer your question, is about 10x what it is in Hong Kong. The big two are Grab and SEA though, which might not be for everyone. Tons of startups and a sizeable Google office (among others) as well.
Taiwan is Taiwan: democratic version of China. I've sung its praises too much here, but it is a superior alternative to those looking to involve their business in the Mandarin speaking world.
Where can I find those communities you mentioned in Vietnam?
Just hit one or two, get in with the local expat crew, and it'll all fall into place from there. Facebook groups sometimes a good way as well. Saigon seems to be the main city for it but Hanoi has crews as well, and they're also spread out somewhat in some of the other cities.
Anyone know what the tech scene there is like?