The only country requiring a visa in South America for US citizens is Brazil. Yellow fever is required if you are going to Brazil from any other country where there is endemic yellow fever, but not sure about the others.
The best way to check on requirements for immunizations is through the WHO website http://www.who.int/country/en/
or through the CDC website: http://www.cdc.gov/travel/
You can usually find this information on the individual country's embassy website (in the USA).