Can I use southeast asian fish sauce as a marinade? I have been using Southeast Asian fish sauce ( its Vietnamese variant called "nước mắm" to be precise ) to marinade meat which I would then fry. From the narrow observation I made though the Vietnamese seem to use it solely to dip the food in it. Is my usage really that unusual?
No, it's not unusual at all. See this page from Vietnamese-American cooks for examples where nuoc-mam is used to marinate chicken, or just type 


  nuoc mam marinade


in Google :)

There are a lot of examples, including fish, chicken, beef, or even mango marinated (as well as dipped, of course) with nuoc-mam (and most of the time lemongrass), and a fair share of them are recipes by 'natives' from South-East Asia, not only fusion food addicts (even if you can be both!).
Some salads like goi tom or goi ga also involve nuoc mam marinade.