My destination is not in Jet Lag advisor

Which city or airport exactly? I was able to pull up some using the Vietnamese spelling.

Hanoi and Da Nang but also tried Saigon

You just need a city approximately on the same geographical longitude, so it does not really matter that all cities are not listed.

As trux mentioned, getting the exact city name is not critical, jet lag advisor will still work correctly. 

However, Hanoi is split up into 4 urban districts - Hoàn Kiếm district. Inputting "Hoan Kiem" returns Quận Hoàn Kiếm, Vietnam.