Maybe fw hard candy + tfa sweet & tangy (or "sweetTart" or whatever tfa is calling it now... I haven't bought it in a while!).
Also, you can get crystalized/powdered malic acid at bullcityvapor.com and/or ecx. Dissolve some of that in PG (I did 1/4th tspn in 10ml, which I *think* is approximately a 10% solution, but you may want to check that. And I have read that most people use a 20% solution) and add a few drops to your liquid for a sour effect.
Easier, there are various "sour" products you can get. TFA has one for sure, by that name. I am pretty sure these products are either mainly or only a premixed solution of malic, but you can always ask the retailer if you want to know for sure.
Citric acid (also available as a powder dissolvable in PG) will also impart a tangy flavor, and in small amounts, can make fruit flavors "pop". I have found that it gunks coils pretty quickly, though.