|I suspect neither is particularly 'bad' for diapers, although I'd be careful with the Vaseline. I used it regularly for nighttime diapers with my first but this was before I knew about the possibility of creams not washing out completly. I never had any problems and Vaseline does seem somewhat more washable than some diaper creams but it's something to be aware of. I definetly wouldn't use it on pocket diapers though, they're very susceptible to build-up and can repel very easily because of it. Honestly though, neither is necessary and it's probably just best to let her skin breathe as much as possible.
As for pre-washing, it depends on the type of diapers you have. If you're using white (aka bleached) diapers then one hot wash is probably sufficient. The point of pre-washing is to remove the natural cotton oils in the fabric since they interfere with absorbancy (it's the same with towels for example, new & unwashed towels won't absorb well at first). White diapers have been bleached and this process removes most of the oils. Unbleached diapers haven't so there's more of the oils left in the fabric. I'd say it takes anywhere from 4-6 hot washes (with just a smidge of detergent, and type of washer doesn't matter) for most of those oils to be gone. You could probably get away with just 2-3 washes though, just be aware that they won't have reached full absorbancy yet and keep that in mind on the odd chance you have leaks.