I dont see your point? Its not because Argentines are poor that Brazil isnt that popular with them? Or because they have the peso. If that was the case Argentines wouldnt be vacationing at all.
My point is that if Argentina and Uruguay were much more developed than they actually are and Brazil was thus somewhat more developed than in real life but not at the same rate at all, Brazil would be a much more popular destination among Argentines and Uruguayans (and would have been for decades) for various reasons. For one thing, middle-class Argentines/Uruguayans would be able to afford more than just their summer vacation in Mar del Plata, Punta del Este, or perhaps Florianopolis. Financial stability (i.e. no hyperinflation, 2001-02 style economic collapse, etc.) coupled with greater general prosperity would ensure that they would routinely make a winter vacation that's more than just skiing in Bariloche or what not. They wouldn't have to worry about restrictions on their banking accounts, they wouldn't have been subject to dictatorship-era travel restrictions in the 1970s/1980s, etc.. That greater amount of such people travelling to Brazilian beaches in the winter would generate ever more demand for more all-inclusive resorts, hotels, restaurants, and other tourist facilities, and more tourists would be attracted to these increasing options (so, a cycle that keeps on repeating itself). Plus, Brazil would be cheap by Argentine/Uruguayan standards (at least up until the current economic boom in Brazil). All of this is no different really from the US/Canada relative to Florida/Mexico/the Caribbean or from southern Australia relative to Queensland, Fiji, Bali, etc.; those phenomena have gone on for decades and not just the past few years. And above all, Brazil is really not far from Argentina/Uruguay, unlike Florida, Mexico, and the Caribbean! As mentioned earlier in this thread, simple airfares to Miami from Buenos Aires are more expensive than those to Brazilian cities from Buenos Aires.
Of course, some Argentine/Uruguayan sunseeking tourists would go to Florida, Mexico, or the Caribbean in the winter, just as some American/Canadian and European sunseeking tourists go to other places around the world (including Brazil) in the winter and some Australian sunseeking tourists go to Thailand and other places in Asia and the Pacific in the winter.
Is it possible that another reason why Florida, Mexico, and the Caribbean are more popular with Argentines/Uruguayans than Brazil is because Spanish is spoken in many of the first-mentioned areas while Portuguese is the language in Brazil - and thus there would be more of a language barrier?