Can I use southeast asian fish sauce as a marinade?

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.

Verb - marinate = to pickle in brine, sea water. Marine.

Noun - marinade = the liquid used as medium to marinate.

Wikipedia on "Fish Sauce":

Fish sauce is an amber-colored liquid extracted from the fermentation of fish with sea salt.

From the definition/etymology of the word of "marinate" - - -> of course, you can. It's your kitchen and your preferences for flavour.

I am sure that marinating only flavours the outer layers of meat and you should cut gashes into the meat to allow the marinade to penetrate.

Rather, your question should be,

how do I marinate with Vietnamese/Thai fish sauce?

I think, in fact, you could even ask the question,

How do I marinate meat with Australian Vegemite or British Marmite?

I think to preserve and complement the fishy fragrance (most people would prefer to call it fishy odour),

• the marinade is retained to be part of the gravy.
• you should have garlic powder and chopped scallions mixed in the marinade (the fish sauce).
• little bit of black vinegar
• I did try mixing in curry powder, cinnamon.

And then the marination graduates into a marinating/poaching process on the same pan by turning the temperature up. What I am describing is a form of poaching which I do at low temp grilling in an enclosed oven.

I marinate at 100F for a couple of hours and then turning up the temperature to poach at 200 F over another couple of hours using various appendices/apparatus to elevate the meat (chicken wings/quarters) to as close to the oven elements as possible without touching them. About an inch of clearance.

And finally turning the temperature up to 350F for 30 minutes to "attempt" to achieve the occasionally surprisingly crispy skin

Turning the meat over at opportune times. The slow grilling can produce pretty amazing crisp on the skin. (However, there was once when the salmon became dry and fibrous after leaving it in there for too long.)

I am sure there is a lot of room for improvement in my culinary timing skills. And I've only tried poaching poultry and salmon. I did destroy a few pretty herring by trying out the same process. And turkey wings/quarters wouldn't even get cooked or amenable to absorbing any marinade under the same process.