I haven’t been to Shanghai yet, so I can’t really comment on that one.
But since you’re going to all three, I’d say each place has a different vibe.
If you’re into gay nightlife and want the easiest, most active gay scene, Bangkok is probably the easiest. Staying near Silom is a good idea since it’s lively and convenient. Beyond gay life, Bangkok also has a lot to enjoy — sightseeing, river cruises, food, spas, and gay saunas.
Vietnam is great too, but it feels different. Saigon is more open than Hanoi in my opinion, and you can still find gay-friendly places, spas, saunas, and great food. The vibe is a bit more low-key than Bangkok, but still very enjoyable.
As for meeting men, Thailand is definitely the easiest in my opinion. In clubs, bars, saunas, spas, and different social spaces, there are a lot of options depending on what you’re looking for. Vietnam is possible too, especially in bigger cities, but it may feel a little less direct.
That said, no matter where you go, I’d still suggest being careful. There are plenty of good people, but there are also bad ones, so just use common sense and stay safe.