Verjus or Verjuice is the very tart juice squeezed from unripe grapes. A juice that is just one step before vinegar in its acidity.
However it contains no acetic acid, but its little acidity is more gentle and balanced than the citric acid in lemon juice
It was very popular in medieval times,when lemon juice was not available.Its flavour has the tartness of lemon and the acidity of vinegar without the strength of either. It can be used as an addition to cooking in vinagrette dressings and when deglazing pans.
So I imagine as a substitute you could use a little lemon juice sweetened slightly with grape juice perhaps?