I would first try the music rights society of your region, which in Vietnam is VCPMC -
http://www.vcpmc.org/vcpmc/
I am not by any means an expert on this, but I think that film makers don't necessarily go to artists to ask permission to use their music, they go to the owners of the rights (publishing company for example).
However, most countries have a Music licenicing organisation such as VCPMC in Vietnam and collect royalties from you on their behalf.