We get this all the time. Customers ask, "How much Vaseline should I put on the skin before putting on the alginate?"
The answer is: None.
The only skin that alginate might stick to is skin like the elbows of an old farmer that are dry and cracked and generally disgusting. Other than that, you're good- you don't need a mold release.
The other part of that questions concerns using Vaseline. Please don't ever use Vaseline. It makes the skin so hydrophobic that its nearly impossible to APPLY the alginate. When you move your hand away from the skin, the alginate sticks to your hand more than it does to the face or torso or whatever. The only thing you MIGHT want to use on the skin is a little water, or a little water based hand cream. Since the alginate is water based, the moisture on the skin makes the alginate sit down very close to the skin. Some say it slightly IMPROVES the fine detail of your alginate mold- most people can't tell the difference.
If you're working with Silicone- please, please, please use petroleum jelly as a release ALL over the skin. Your model will be VERY disappointed with you if you don't. With alginate you can safely give it a miss.