I'd go with Caracara, I've seen them soar with vultures along the border in Arizona, which osprey wouldn't do. Caracara and vultures have similar eating habits, they're scavangers, while ospreys are predators - they like their meat fresh.

So now you'll just have to pay closer attention to the soaring vultures to see if it's the Caracara, a turkey vulture, or a black vulture. Need a good bird guide (book) or binoculars?