Rice vinegar and soy marinade for black cod?
Most of the recipes call for mirin, white miso, but I don't have those. Could I make a marinade with rice vinegar, soy sauce, sesame oil? What proportions would work and how long to marinate?
Vinegar won't marinate the fish... it'll cure it. You'll end up with a sort of Asian ceviche. Which may not necessarily be a bad thing, but I don't get the impression that's what you're hoping to achieve :-)
Soy sauce will take you in the wrong direction--way too salty and stinging. Mirin and white miso are delicate and almost sweet flavors, plus the mirin caramelizes when you cook the fish. You might try marinating in Sake with a little sugar and salt, but it's a very rough approximation. Another option is to try to find a mail order source http://www.uwajimaya.com is a good start. I'm guessing from your handle that you live in NYC--I'm sure there are lots of places to find those ingredients.
re: Erika L
Honestly, it's so wonderful with miso that I wouldn't do it any other way. I bought a small container of miso paste from my fishmonger, and mixed it with mirin. You can find both at an Asian grocery, and I'm sure you can find mirin in the Oriental/Asian section of any good quality market these days.
I marinated the cod in it for about four hours (you do not have to do this overnight and certainly not for two or three days.) When I cooked the fish, I sauted it first flesh side down, then I flipped it over, put some more marinade over it, and placed it in a baking dish with about 1/2 inch of hot water. I put the dish in a 425 degree pre-heated oven, and roasted it for about 10 minutes.
All this depends on the size and thickness of your fish, of course.