In addition to the above. Thailand is most touristy and fun/party style. Vietnam is much more authentic very nice people and very cool architecture blend of French and Asian.
Thailand is a monarchy and the king is a very central figure. It is also very religious with tons of temples and statues (big problem because it is real עבודה זרה unlike churches etc that we have in Europe). Vietnam is communist (to a certain extent) and I don’t remember ever seeing people making קרבנות.
I spoke to a black guy in Vietnam (head of Kenya airways) who said he was touched by different people while walking on the street who thought that he had paint on himself 🤦♂️ I also had my beard touched a couple times while walking in the market from farmers who had never seen someone with real facial hair... (you have to understand the culture to realize that they are not rude rather have a different way to interact with people/things and might have never met people who aren’t SE Asians)
This would be very unlikely to happen in Thailand (at least not somewhere where you would be going).
Chabad and kosher in Thailand is much bigger, so is general tourist infrastructure, hotels, resort etc. But Vietnam has also plenty. There’s large Chabad houses in four cities in Thailand with restaurants etc. There are 3 Chabad houses in Vietnam but they’re smaller
The truth is that after all this, what will really matter is which country will be open by that time and I don’t think that you will actually know until very close to your trip...