I just had this issue, it was a problem with the address on the visa application, not matching the billing address of the credit card. It can be solved by either using a different credit card or re-processing a new application. I spent an hour with my bank (Chase) to learn that there’s nothing that can be done on the banking end, if the vendor is trying to process a charge with the wrong billing address, it’s auto declined. That was my experience, hope it helps and good luck. 
I’ve been on tourist visas for 6 years straight, then Muay Thai for the last year. Going to apply in HCMC in 3 weeks, I don’t anticipate any issues (many friends approved with similar circumstances). Has anyone in a similar boat had any issues with obtaining the DTV?
