There is a clinic on Libertador approx Mendoza that does vaccinations -- yellow fever you won't need a prescription. Tetanus, you may need a prescription. Tetanus shots you can get at the pharmacy as well though -- just ask if they do inyecciones there. If they tell you that you need a receta, you can go to a clinic with a guardia and get a receta.
This is the place we went for our shots, make sure to get the receipt for the vaccination as you may be asked to show proof of vaccination at the border:
Dr Stamboulian Centros de Vacunacion: www.drstamboulian.com.ar