How to reduce marinade to a sauce?

I know that this is a common method to make a sauce from a marinade but I haven't done it much so wanted some tips. Usually my Friday night consists of seafood marinated in an Asian-style marinade with green onions, garlic, rice vinegar, soy sauce, sesame oil and ginger. Usually I just add a little water and pour the marinade over the finished fish, but last week decided to try out the simmer method to make a sauce. I simmered on low and the smell of evaporating vinegar was quite strong and it seemed to boil very readily even though on very low heat and I backed away in fear. Should I just continue to simmer/boil to reduce the water? Any issues with vinegar in the marinade? If it includes a high sodium ingredient like soy sauce should I worry that it will become too salty when reduce?