This post is easy-you said it already, try some white balsamic or pear balsamic vinegar, and lemongrass,ginger,shallots,garlic.....fish sauce is an aquired taste(quite unique salty PUNGENT taste-but some commercial versions for restuarant and supermarkets are less extreme)I have access to some green Thais now and next year will have Thai Fires in my garden again.
I have notice the friends and customers I have that are from the Thai region love their vinegar.
I am thinking some Thais, garlic, onion, white or balsamic vinegar(maybe both), ginger, Jimmy Nardello sweet peppers, some limes, and for more heat some Fatalli or Naga Morich or possibly Naga Jolokia(first 2 I have a few frozen still and the 3rd I have to check if I can still get in from Holland). The fish sauce is.....I don't know. Like I said I am going to make an effort to get some Thai food this weekend and that will help me put together a better desired profile.
but I'd be looking for a savory type sauce -so something salty to "savorize" the sugar is called for.............my 5 cents.