Are there any side-effects when you use Vaseline on the skin?

a lot of people are allergic to petroleum jelly, which is what Vaseline is, so yes, there can be side effects. Also it is actually drying to the skin, long term, so it's slightly counterproductive to use it as a moisturiser. It is also not suitable as a sexual lubricant if condoms are also being used as petroleum jelly damages latex, thus rendering the condom unsafe.