It was in a Sage (HAH!) Advice.
Yeah, Sage Advice is pretty much worthless at this point. I mean, for pity's sake, casting
revivify on a destroyed zombie brings the creature back as a zombie? What gives?
Anyway, to the OP... by the book, a giant vulture can carry 450 pounds (Str 15, x15 pounds, doubled for Large size). If the combined weight of the PCs
and their gear exceeds 900 pounds, then even by the book, the vultures can't carry them. If you wanted to be stricter, you could apply variant encumbrance, or just house-rule that a flying creature's carrying capacity is half normal.
However, I would suggest a different approach. Allow the PCs to bypass the first few encounters--and make sure they
know that they have successfully bypassed those encounters, so the druid gets to feel clever and cool. But then they get attacked by aerial monsters--and as the enemy approaches, the PCs hear the leader scream, "Kill the birds*!" (This should be a couple of rounds before battle is joined, so if the PCs are flying high, they have time to start descending.)
Your biggest challenge here will be avoiding mass PC death. Giant vultures have AC 10 and 22 hit points; they aren't exactly hardened targets. I'd be generous allowing PC shenanigans to mitigate falling damage. If you can figure out a way to get a
scroll of feather fall into the hands of the party wizard ahead of time, that would be ideal. The end result should be that the PCs get a significant benefit out of the druid's spell, but it doesn't let them bypass the entire adventure.
[SIZE=-2]*If the players complain about this, you can point out that it would have been worse if the monsters targeted the PCs. One failed Concentration save from the druid, and the whole party is headed for the ground at high speed. The sky is not a safe place for low-level adventurers.[/SIZE]