Any recent experience on whether the vendors at the basecamps prefer USD, or the Argentine peso?

I haven't been to aconcagua for many years, but be aware that the current official value of the peso is 6.30 to the dollar and the black market value (usually called the dolar blue) is 9.55 to the dollar - the ATMs will only dispense pesos and they will charge you the official rate - the vendors will almost certainly pay better than that for dollars, but will probably offer far less than the blue rate

your best bet will probably be to sell your dollars on the black market in mendoza and then buy directly with pesos at base camp - i imagine you can use the same strategy to pay for your permit - legally, they are required to accept pesos at the official rate even if the prices are quoted in dollars - some towns are easier than others in terms of the availability of safe, convenient, reliable money changers, but usually it's not a big deal - somebody at your hotel/hostel can probably point you to the best spot

be aware that this is general advice for anywhere in the country - plaza de mulas may have a slightly different culture because it is such an international spot - perhaps somebody who was there last season can chime in with some more details - any information from prior to last season is undoubtedly outdated, as argentina has changed considerably in recent years

also note that if you have extra pesos at the end of your trip that you want to convert to dollars, you will have to pay the blue rate, unless you have a receipt showing that you initially bought them at the official rate

Thank you for a thorough explanation of this market value vs. actual value issue. I leave on Thursday to head down there and have been hearing about the black market value and how much higher it is for months now. I'm hesitant myself to get the better rate as I will be traveling alone and my Spanish is very very poor, but it is somewhat tempting.

OK, so I answered my own question. It is safe and easy and you really do not need to know any Spanish. They will have it on a calculator for you or a phone to see before exchanging. Very easy. Current rate on the street is 9.3 give or take. Leaving the hostel in an hour..... wish me luck and pray the mtn gods are nicer than the baggage gods at United. Needless to say I have had a few extra days in Mendoza. Albeit it is a nice town.

Hope to give everyone an update once I am down.