Even better go for a holiday in Danang city, central Vietnam. A beautiful beach city that’s much nicer to stay with a family than Hanoi. You apply online anyway so it doesn’t matter where in Vietnam you are situated. Sorry to hear about the rejection. I’m sure you will get there. Some really unhelpful stupid comments on this page. Good luck!
There should be a fee for the classes, maybe a small fee for the admin/documents. Plus, if you are applying the visa yourself online, that fee on top. That’s it.
The Muay Thai gyms are gonna be busy. Usually it’s the gyms run by western owners that are better at the DTV. Because foreigners have to have their paperwork together so they should know what you need. Contact Tiger Eye Muay Thai. Bangkok. They are great and very professional