My wife is Paraguayan and I have visited Paraguay many times. Paraguay has some problems with Dengue (which is somehow related to yellow fever, but I'm not very knowlegable on the difference). However, most of those problems are in the country and in places where the land is not well kept and mosquitos are allowed to flourish.
Given how vaccines can be dangerous, and the fact that I have never had a vaccination for yellow fever to go to Paraguay (and my wife's family lives int he deepest backwoods country) and know no one who has had the vaccination, and know no one has contracted any illness, much less yellow fever, I would have to say that getting a vaccine is very unnecessary.