it’s a multiple entry visa. You can come and go in ten minutes if you like. There are other multiple entry Thai visas which people bounce out/in with by just walking across a border and walking back in again. There’s no ‘I need to make this look reasonable’ like it’s a genuine trip or visit. You can use the multiple entry aspect of it exactly how you want.
If I’m in Phuket, for instance, and I have any kind of multiple entry access to Thailand (examples of which are; multiple re-entry permits on an extension of stay, a multiple entry tourist visa, and the, seemingly discontinued, multiple entry one year Non-Imm O) I could decide I want to go shopping in kuala lumpur for the day, fly out in the morning and fly back the same evening. There would be no questions as to why I didn’t stay for the weekend, or what was the purpose of my visit to Malaysia. It’s a perfectly normal thing to do, it takes the same amount of time as flying to Bangkok, and if your current visa or extension allows for it, you can easily do it. People fly to Singapore for the day quite regularly, it’s under two hours.
I think you’re inventing restrictions and limitations on this visa that don’t exist.
As for the cost, there are visa run companies charging around ฿6000 for hours in a van whereas a same day return Phuket to KL on Air Asia 08:15 outward, 19:35 return flight is less than half that.
It’s just proof that you’re in Vietnam whilst applying for the DTV
What people usually provide as that proof is; the entry stamp in their passport from Vietnam immigration at the airport. Their boarding pass from the airline, and their accommodation proof, which an AirBnB booking/receipt is.
If you are in the country for 90 consecutive days you report that fact to your immigration office (your TM30 designated which office is ‘yours’). That is what a 90 day report is.
If you’re not in the country for 90 consecutive days, then you don’t.