People have been eating Vaseline for its health benefits since it was created in the 19th C.
It does have one benefit: It's a laxative.
It does not cause any health problems by itself, that is, it is not toxic. But as with anything, it can be overdone. No sound diet can be based on using laxatives. Vaseline can, in large quantities, coat the inside of the intestines, killing the bacteria there and certainly blocking the absorption of nutrients. This will cause weight loss because it literally starves the body, but not in a good way.
Because it will give you the runs, anal sex will be even more gross