As far as I know the only one required by law is Yellow Fever, but to be honest I wasn't asked for my certificate in any South American countries. The minimum I recommend are Yellow Fever and Hep A. The more basic you intend your food and accomodation to be, the more likely you are to need extra jabs. Make an appointment with a Tropical Medicine clinic but don't let them scare you into taking every vaccination under the sun because it is extremely expensive and often unnecessary. Do make sure they give you a kit for tummy trouble, as it's very likely. And stay away from mayonnaise in Peru, I got food poisoning :o( It's a fab country though!! Enjoy!!