They have told me they are moving to Evisa in October and so there are no more online appointments. I have talked about this a few times in the group. Hanoi as far as I know still have appointments but right now you will not be able to do it in HCMC until they announce the Evisa process.
I just did it in HCMC and I think if they give you the slip and take your money there is a good chance you are fine. I cancelled my Ed visa before doing it but when I entered BKK the IO asked me do I want to come in under the Ed visa or the DTV. I thought this was odd because I was told you CANNOT have 2 active visas...when I told him the Ed visa was cancelled he looked again and saw the cancelled stamp on it. If I were you just cancel the other one when you get back.