Not a Flavour i'd particularly like to vape myself, but perhaps i can offer a little insight.
The thing that gives sherbert lemon it's sourness is likely Malic Acid.
This can be bought cheaply as a powder, and quite possibly can be mixed with VG/PG. It's freely soluble in Alcohol and Water.
If you can find yourself your preferred lemon flavoured liquid, you can try adding this to give it that desired tartness.