what do you mean "Border run costs are starting to be apparent too"? That still looks like the easier no-brainer solution to me. To each their own though. If you like paperwork and bureaucracy more power to you. I don't.
You can leave Vietnam if you've already submitted your application. Plenty of people have already done what you're asking. It's UNREASONABLE for you to be expected to remain in Vietnam when you don't know how long your application will take to process. Just leave, and then reenter Thailand when the DTV is approved.