I know last year when I looked into it we Had to get them. For sure for Brazil. I Think Every country is different. I would deffenately get them. If you can. Another problem would be getting out. I heard you cannot leave brazil to the US unless you can prove your vaccination. Not positive this is accurate.
Pura Vida, hope this helps.
Last year I arrived in Rio from the UK via the USA. Nobody asked for my yellow fever. Then proceded to spend 6 months traveling to (in order) Argentina, Uruguay, Argentina, Chile, Argentina, Bolivia and then finally Peru, and home via the USA.
I was NEVER asked for proof of vaccination. I would recomend it but I doubt you will be asked.
You will have no problems getting into peru without the vaccinations, didn't get asked moving between countries in south america, but did get asked upon returning home to australia. There are certain regions that are on the list. parts of colombia, bolivia, brazil and argentina is what I was asked last year, but I believe it changes with seasons/outbreak areas.
There are doctors in lima that your hostel will be able to recommend, hopefully they will be able to help you out.
The only place in all of south america where they have asked my YF vac was arriving in Brazil at Corumba (Pantanal), but not even any other brazilian border, not even during the YF outbreak in parts of Brazil beginning of this year (2008)