I was in the same situation a few years ago - I really wanted to have a Vietnamese beer mug just for keepsake. I guess you need to have some luck because they dont usually sell them.
I got one Tiger beer mug cos my cousin owns a cafe and they get free beer mugs there (to advertise Tiger of course). If you know someone who works for these breweries I'm sure it would be easy to get one.
Also one time in Halong Bay when I was drunk I managed to steal an Anchor beer mug from a bar - wouldn't recommend this though...
