Applied and got approved with Non ED. Not sure if the same will apply to Non O. However, upon entering, the DTV was what was used. Not the other one. I'm not sure you can choose which one to use. Your DTV is valid from the time it is approved so it I don't see why you will wait til 2025 to use it.
This is not the right or legitimate way to do it. Go to the country where you are applying from. If you are going to Vietnam why not apply from Vietnam.
You don't have to apply in your home country. You should be able to apply in neigboring countries. I have a PH passport. I applied and got approved in Laos. Suggesting that you have to go back to the US can cost both a lot of time, money and effort which in my opinion is not worth it.