Vaseline is not a good thing to get into the vagina. It is not water soluable, meaning it's going to stick to the insides of the vagina potentially trapping bacteria, etc.. Always use a water-based lube. Also Vaseline will break down condoms.
You should avoid vaseline and use a water based lubricant like KY-Jelly or similar. This is especially true if you use a condom as a petroleum based product (like Vaseline) can deteriorate rubber and cause it to break.