Sounds like you got a multiple entry visa in Vietnam. Have to leave every 3 months. Inside of Thailand you just get an extension on that visa you got outside of Thailand to stay longer in Thailand. It is this extension you need to show the cash for and you don't have to leave the country every 90 days.
? Is it because they don't know or just scared to get out due to current situation. My friends elderly parents haven't been out the house since March. I'm sure they would have jumped at the chance if they had known.