You can overstay by two days without a problem. You will just pay the fine when you leave.
I arrived today (USA passport) and was asked when I was leaving and I informed them in 32 days… And I asked “will this be a problem ? … or will I need to do a border run before then”?
They said no problem just pay the fine for two days when you exit
Yes, that works perfectly for Thailand. But please be advised you need a letter of approval for your vietnam visit. It’s easy and it can be done online.