Why is my puppy throwing up and feeling sick? When to see a vet?

Hello, and thanks for using Petco Pet Education Center, formerly Petcoach. The licking of other dogs' private parts is a gross habit, but probably not the cause of the vomiting. Causes of vomiting in a puppy can include dietary intolerance or indiscretion, food allergy, intestinal parasites, GI blockage, gastroenteritis, bacterial or viral infection (including parvo virus if not completely vaccinated), among a few others. If the vomiting lasts longer than 12-24 hours, or if Prince develops other symptoms, such as diarrhea, lethargy, abdominal pain, etc he should see a vet as soon as possible. I would only offer small meals of Prince's normal puppy food, or small pieces of boiled chicken breast in this period. If he isn't interested in food, it may be a sign that he is very nauseous and he should get checked out by a vet. I hope he starts feeling better soon!