Possible substitue for vinegar
Citrus juice is probably the most widely available substitute.  However, if you can get your hands on some Verjus (or Verjuice) you will be pleasantly surprised.  

Verjus is the juice of pressed green grapes.  It is similar to wine but the grapes haven't had a chance to ripen and there isn't any alcohol.  It has a much more neutral flavor than lemon juice.  It can be difficult to find though.  I buy mine at a local vineyard.  You may be able to find some online.

https://en.wikipedia.org/wiki/Verjuice