Well you did scientifically think of vinegar but only afterwards :-) I have used white wine vinegar when I didn't have any malt vinegar and I don't think it affected the flavour much but balsamic might.

Make sure the sugar dissolves completely before heating it to turn it into toffee. The temperature will affect the set/softness/crunchiness but isn't disastrous if you haven't got a thermometer. I've never lined the tin, just greased it. It can stick to the tin a bit but you just turn it upside down and bash it and I'd be worried the paper might stick to it otherwise. Good luck. It is fun edible chemistry!