Depends on the person, is he really a friend, a social acquaintance or a colleague. Ive been to parties where people gave gifts, a lot of gift certificates actually. The latters not my thing but it does seem to be popular around these parts. Ive also been to parties where no gifts were exchanged.
Perhaps play it safe and go with a consumable...i.e. food or wine gift.
Since your Vietnamese...if he's adventuresome and likes to cook, Id get him a gift basket representative of your culture. A picture book on vietnam, 1-2 easy vietnamese recipe and matching key non-perishable ingredients (bottled sauces or herbs and such...not pho, to hard... and/or if you can get your hands on it some vietnamese beer or wine. Dont forget to include a note on how to say a few words in vietnamese---i recommend pick up lines if hes single -- then have a good laugh watching him get drunk and telling ladies he loves them in vietnamese all night.
If thats too much trouble a bottle of wine should be fine.
hth -- have fun
PS: if you make summer rolls, send some my way...my one and only batch turned out lumpy

PPS: If you have one, could you please post up your favorite Vietnamese restaraunt in Munich. Ive been to one or two but I know theres a few more in town.