I got this to work with ITA Matrix one-way, WAS-IAH-BUE-VVI with the 15-hour day layover in Buenos Aires. In theory you could get off the airplane in EZE and throw out the VVI leg on Aerolineas, and just treat this as a one-way to Buenos Aires.

It appears that US citizens can get the Bolivian VISA on arrival (but it still cost $160), but you also need a yellow fever vaccination for Bolivia, which the United CS agent should be checking for at your point of origin. Timatic doesn't mention this, but the state department website does.

In Matrix, I searched leg 1 WAS to BUE and forced United routings (C:UA+) and looked for P fares (F BC=P). This should exclude Copa segments. Leg 2 I searched for BUE-VVI and left the controls blank. The problem is AEP-VVI does not operate daily which restricts the days you can depart Washington. If someone else knows any fare-rule hacks for this, let me know, but I'd take a 777 IAH-EZE for 400 bucks and call it good