I'd love Allan. Any time I've seen him play, he's ferocious. And surprisingly we're lacking a player like him. We have 4 box to box midfielders, and a regista. We don't have a destroyer. Unlike Vidal, Allan is defense first, so I'm sure at the minimum he'd fill a need (assuming we kept both).
I also think he'd be reasonably priced. Vasquez I'm more skeptical. He's good on a provincial squad and he's 25. It would be nice if he panned out and declared for Italy because lord knows we need an attacking midfielder. Our fantasista prospects are at an all time low.
For Vidal on the other hand. We'll get 20 million for him now max (given his injury concerns and form). Worse, he could let his contract expire and leave on a free. As far as I'm concerned with Vidal, right now, putting Pereyra in form him is not a big deal because this isn't the Vidal we know. It might sound extreme, but if he still has remnants of the injury, I'd rest him until he is better. I'd also get him on an extension as soon as possible. I've had a knee injury, and rest is the only way.
Also on Vidal, given his injury, a change in coach/tactics, the manchester united ****, it's not surprising to see him play worse. The priority has to be taking the prudent approach in letting him recover (if that's the issue), and getting that extension penned.