Covid was plessing for me. They didn't care how long you've stayed and I just ran through immigration. 24 years of history being here with all sorts of visas and exemptions.
Once I even flew to Phnom Penh from Europe and took a bus to Thailand. Was so sure that they'll not let me in.
Now I'm 50y , so don't have to play the system anymore. The only good thing for coming old.
Phnom penh is not good place to get the visa if you have a stamp or 2 in your passport. I know only 1 or 2 agent that can get you the visa there. This was 7 month ago
Make sure that you can return to Vietnam if they denied you entry to Thailand. Otherwise you'll be flying back to home.