My suggestion would be to use the benefit of the different award allocation between UA and Varig.

Varig opens up, up to 4 seats/cabin, way ahead of time. UA may open tons of award seats but when date approaches.

Suggestion: secure your seats with Varig and swap them for UA as date approach, if UA is your line of choice. This way you have your travel secured in the cabin of your choice, yet free to change carriers.

Take the benefits of *A, which are the same number of miles, regardless of carrier flown - change them at your will until you start using your award ticket.

Editted to add: Odd that reservation you were given. On Varig their first suggestion should be LAX-GRU-EZE as they've many more flights out of GRU than GIG. GRU is currently Varig´s preferred hub. If you will please check that your going back to GIG is not just to pick a direct flight that will, nevertheless, stop in GRU on its way to EZE !!! Also, I don´t have it here with me RG´s timetable but I guess the only flight that may have three-class F seats on that route is their one MD-11 daily flight. All the other many flights are sold as business. So, that leg you´d have to downgrade to C, which I say is not bad at all.