I'm not personally one to add sweeteners to anything as they can sometimes (for me) mask the true flavor. Most flavorings come off plenty sweet enough (to me) without adding them to the mix.
For sweet tarts, you may want to look to TFA for their Sour "flavoring" to boost the sourness of the final e-liquid. Some use it in drops, others up to and around 1-2%.
Thanks for the help I found that at 10% I get a really strong flavor it almost feels like it's masking a chemical taste though maybe I should go lower the butter toffee tastes extremely similar to the carmel coffee werthers hard candy at 10% and pink square for me at 12% has a pretty strong flavor I get a lot of cream from it though what I was really wondering was what are the best flavors to buy so I don't need to do the runaround and buy a bunch of bad flavors I wanted to try the cant crunch berry also heard the skittles was good they have a bunch that seem good but I'm not to sure like snickers