When they are nursing on themselves, it can become a really bad habit and they can start sucking the fur off and creating sores etc...Some experts look on this behavior as feline OCD Obsessive Compulsive Disorder and prescribe mild mood altering drugs for the cat. Other people say that you should try to give the cat a substitute item to suck- like a small stuffed animal, or add quality food to their diet and see if it is nutritional in nature. My vet believes, and I agree with him, that is a nervous disorder and has nothing to do with being abandoned early. I have had a lot of orphaned kittens under my care over the years and I have only seen this type of sucking behavior a few times with the kittens that come through here. I would try the substitute object, and add some quality canned food to the kitty's diet to see if that helps. You can also use feliway plug in room mister to help if it is a nervous disorder, it might.
But please don't use vaseline or salt water on the area, it will exacerbate the skin, and cause more problems in the long run.