Well... the intention of the form TM.47 (aka 90 day report) is that the Immigration can check if your registered address is still "up to date". So... it makes kind of sense that you have to report in the province where you live, no?
Correct... it totally depends on the officer you have to deal with... some know about the agreement, others don't. And from the some who knows... well... let's say their memory is not the best sometimes... 😁
You are right... I forgot to mention the needed IDP in combination with a Thai license. Although Vietnam "officially" do accept the Thai license / the ASEAN Agreemnet, they don't do in real life.
Yes, your license is valid in all ASEAN (Association of Southeast Asian Nations) countries. These countries are: Brunei, Cambodia, Indonesia, Laos, Malaysia, Myanmar, the Philippines, Singapore, Thailand and Vietnam. And, of course, only for what it's issued... so... just motorcycles.