You know this for sure? How do you know this? Some countries only allow residents of that country to apply for the DTV. Korea is an example of this. So are other countries. Can you show evidence that non-residents of Vietnam can apply in Vietnam?
Extension for other visas is 1,900 baht. No one has ever extended a DTV and so no one can give any definitive answer on that. However, we can safely say that the 5 year multi-entry DTV visa is 10,000 baht. It mentions nothing about a 10,000 baht extension.
you have 0 evidence to back this up so don't say it wont happen if you get the visa since you have 0 idea if that is true or not. Remember, immigration is a separate entity and they can do whatever they want and ask for proof of enrollment in your school / program when you try to re-enter. The fact is no one knows the answer to his question.
I'm in the same boat. Shoot me a message if you'd like. From what I understand you need to cancel your Ed visa and leave Thailand before it expires. Then apply for DTV outside Thailand and come back.