FA mad fruit ( mad mix ) is a authentic red bull, but it still leaves room to add anything else you want.
Now the question is sweet and tart, as in the impression/sensation? If so you got the right flavor, start low and work up, by itself that flavor at higher percentage becomes very bitter and sour, not really tart. It has a raspberry note to it as well. Doesn't work very well with beverages, its more for candy type of vapes like sour worms, gummy rings etc. Only beverage it kinda works well with would be koolaid.
If you meant the candy sweet tarts, flv candy roll would be my first pick. Again start low and work up.
But I'm sure it would need more additions, but maybe that's the flavor direction you were looking for tho.