Some thoughts.
To get a really good sweet tobacco flavor, what do we have have to add? Just starting with a tobacco flavor or a mix of tobacco varietals and adding sweetener is not likely to produce a yummy sweet tobacco juice. We need a flavoring or flavorings other than tobacco to "carry" the sweetness.
One candidate is vanilla, but I think we can reject that one as a single flavor solution because really sweet vanilla becomes almost sickening. We might include vanilla in a mix of extra flavors, but I don't think it passes muster by itself. No, the obvious choice is caramel, which has sufficient depth and body to provide a foundation for the sweetness, deliver it, and still taste great.
So, we're probably in the realm of caramel tobaccos here, which with vanilla might include RY4s, but I think probably doesn't in the real world. RY4s that are extremely sweet never get a very high grade from me for some reason. They just don't taste as good as RY4s that keep the sweetness well within bounds.
We might also include a bread/dough/cake/cookie/danish/crust component, which could partner with the caramel in providing a solid carrier for the sweetness. Vanilla would be integral to the dough/cake element for flavor and smoothness. This would be the rough equivalent of adding tobacco to Gooey Butter Cake. Vape-a-licious' StL Gooey Butter Cake is almost ridiculously sweet, so adding a good tobacco anchor to that one might suffice. Lord knows we'd have a world of tasty straight tobaccos from which to choose.
NLV Gold and Bounty Hunter are good examples of this general mix of ingredients, although I agree wholeheartedly with gthompson and Mr.Mann that both those are "ghost tobacco" juices. They do have the caramell-y cake-y thing going on, but the tobacco is more a mystery element just adding some much-needed body. Another one with more obvious tobacco is VermillionRiver Kentucky Vanilla or Kentucky Maplewood, since the Kentucky Premium blends all start off as caramel tobaccos, and they're pretty sweet. We'd have to add more sweetener, of course.
Another candidate might be Caramel Spanish Gold from Sweet-Vapes. No doughy element in that one, but very deep caramel combined with a good NET tobacco. It's not sweet enough as sold, but that could be remedied very easily. Add acetyl pyrazine or vanilla cupcake flavor along with a strong jolt of sweetener to Caramel Spanish Gold, and we might have a winner.