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There is no embassy of Nepal in Vietnam. The closest is either bangkok or burma/Myanmar. If you can travel there, you can get visa from there. However,it is also possible to get visa on arrival.
For more info please try http://www.welcomenepal.com/ or http://www.mofa.gov.np/
I dont think you need a visa for Nepal, I got there and get a visa on arrival for 20 bucks. it is pretty easy actually.