That's insane. You are supposed to apply where you reside however nowhere it says you cannot leave the current place of residence, in this case Vietnam.
(entering Thailand while waiting for a visa is another topic)
Greg Alexander fair enough. Thank you clarifying. Although I read stories of people not needing to show the visa. Maybe after first use it shows up in their system.
"because this is the only proof that you have been issued a DTV." - well no, it's in their system. The fact that they demand to show your visa is another story.
let's talk about how they don't have validation on their own OCR input. You cannot manually enter 0 in the name and yet when you submit passport and it misreads O for 0 it is somehow your own fault.