Hi Lynda,
I had a yellow fever jab four years ago as I was doing the South Africa/Zambia tour. I did Chile, Argentina, Brazil the following year and there was no mention of needing the jab (but that might have been because I'd had it the previous year!). However I do not remember anyone else on the tour mentioning the need for it. Check with your local medical centre at least two months before you travel to see what jabs you need. If you are over 70, the yellow fever jab is normally not advised. Certainly there is no need for a yellow fever cert to visit those countries, so I don't think it's needed, but better to check just to be certain.
Hope this helps!
Best wishes,
Bob
PS the South America tour is amazing, one of my best holidays ever so I hope you go and have a great time!