Hi,
Vaseline is not harmful to dogs. He will probably have diarrhea and this can be a bit of a pain for you. To help him NOT do this....it would be helpful if you could feed him a bland diet. Make a nice mix of boiled chicken and rice, and use about a 50/50 mix. Rice is a binding agent and helps with "potential diarrhea" and chicken is a protein that is very easy to digest. You may want to give him some bread as well. Just a few slices. This can help "pull" the vaseline through the system. If he does start to have diarrea, you can give a dog this size 2 tablets of immodium. Only do this once. If he continues to have diarrhea after the immodium and the rice and chicken, he needs the vet. At any rate, do not let him have diarrea for more than 48 hours without vet care.
I hope this helps you "head off" your problem!
Good luck!
Dr. Pat