Peopl use "vanilla" pejoratively all the time, but I don't think it is supposed to be used that way. "Vanilla" means that is support whatever flavors you decide to make central to your sundae. Maybe you like fruit, or gummy bears, or jalapeno. Vanilla lets you have those without overpowering them like chocolate or something like Rocky Road would. Vannilla gives you control over the flavors.
Among the listed choices, Dragonlance is actually a good example of how you add toppings to vanilla (either of the other two, IMO) to get a specific flavor (in this case, Mormon Myth inspired fantasy). A "flvor" like Dark Sun is too powerful, too distinct to allow for such a thing. That's why, I think, FR is so popular: it is easy to make one's own by choosing your bad guys and your conflicts.