But here is the thing about Flacon, he is a supporting character, he is neither the main protagonist or the main antagonist.
Vulture as the Big Bad of a film, would drive the plot much more than Falcon would in the Captain America films and a compelling Big Bad needs more then to be gimmick and an excuse for some aerial battles (which we have already seen in the past films with Green Goblin), a compelling Big Bad actually drives forward the main plot of a film, so you need more then what people are describing Vulture as in this thread. Vulture at the least, needs a scheme that is more clever then robs banks.
The thing I like about Mysterio is he would provide a psychological challenge to Spidey, rather than a physical one and I don't think Vulture is a very impressive physical challenge, so I think you have to pair him up with a more physical villain and focus on Vulture's cunning, if he were to be the Big bad of the film.
I also don't think the old vs. young theme is interesting enough in of itself to instantly make Vulture a compelling villain, you need to add more into the mix.