Well... it's not impossible nut i think it is more likley to be coyotes or something and the vultures are just there eating what the coyotes are leaving. I'm thinking if it were an arieal predator you'd find feathers or something. I don't see how vultures could carry off that many full size ducks that fast.
Coyotes have been bad here this year. Lots of folks losing birds and a friends little dog was attacked one evening just a few feet from their front door while it was out for a potty break.
I have tons of turkey vultures at my place too. So far I've never saw them swoop down in an attempt at the duks.
But again.. anything is possible i guess.
Editing to add hugs.

So sorry about Limpy and the rest.