Vietnam has no seasoning requirement. A final balance of 500k is needed. Book a course at least 6 months long. Check the main DTV group on FB 100s of tips from applicants from Vietnam.
There are travel agents that can arrange it. Klook, Viator, Expedia offer the service. There are also fastrack online services you can book direct wit. It should cost around 2000 THB, a bit more via some agents. I have used it before when arriving in rush hour at the airport and needed to get into central BKK very quickly.
You can find 1000s of posts from last year to this week which detail applicants approved in Vietnam with just a final balance of 500K. There is no seasoning requirement in Vietnam