Just received my DTV went to Vietnam to do the process as I was already in Thailand.
Whole process took six days I opted for the Muay Thai training package. I went through muaythaiboxingvisa .com
Just had to upload bank statement and the documents from the Boxing camp. A friend of mine had already used them and was recommended to me and the service was very good and there are always on WhatsApp ready to answer any messages and questions.
You arrive in Vietnam I just had to send over pictures of the entry stamp and upload passport photo .
The Boxing option I think is good in that. You don’t have to give too much documentation to do with work as you will be in the country training and all you have to supply is bank statements. .