Unemployed applicants have been approved for the DTV. How long was the Muay Thai course? You can apply right away at another embassy, Jakarta, Laos are good options. Triple check your application before applying again. Did you include a cover letter detailing your long term need for the DTV via Muay Thai?
There are delays in Vietnam at the moment, Cambodia too Using an agent won't speed things up, they only do what you would yourself do. Hopefully you get your approval very soon.
I would Just provide the balance from that active account in addition to the 500K account to emphasize your financial independence. No need to explain where it has come from as you are not seeking DTV sponsorship.
Overstayers have been approved for the DTV. 500k should be in a personal bank account. Vietnam is among the top embassies to apply from which speaks to the ease of its' application process. London is also very "friendly" with its' application process.
Anonymous participant Yes, just be patient. You won't have to wait as long as you have to get your DTV, which is the good thing. Explore Cambodia while you wait, it's a nice country.