Here is the video we took of the congregating vultures a few weeks ago. (Don't quit it early -- the numbers keep growing!)
Because nothing in my research addressed this behavior (I could only find info. on vultures congregating when roosting), I contacted the Turkey Vulture Society for information on this behavior, and below are two responses I received:
This from Maria del Mar Contaldi:
Turkey vultures soar taking advantage of the thermal updrafts, and when this birds congregate in roosts, sometimes of hundreds of birds, they may all "ride" the thermals together. In particular, certain populations of this species migrate in autumn, the western populations go to Northern South America, they are counted by the million at the watchsite in Veracruz, Mexico. The Eastern populations don't leave North America but they do undertake short distance migrations, and they do so in flocks, big groups. This may be what you are looking at, yet it should prove useful if you could write down exact dates of arrival and departure and see if you notice any wingtags, some study groups band the birds to follow their movements, and study these patterns. There is a lot of information being gathered about it, perhaps your next search should be about the migratory movements, you will get a lot of data about it in the internet.
This from Brandon Breen:
It is generally believed that the vultures do this to both play and socialize. Humans aren't the only organisms to play and just enjoy certain behaviors, and it is possible that the vultures enjoy flying and will do so in the evenings after the foraging 'work' is completed. Also, these flights are not energetically expensive when the air movement conditions are right. That is to say, if the birds are able to soar (not flap) then they're not using much energy so the cost to this behavior is minimal.
There is a lot that is not known about vulture behavior and social structure. Nevertheless, vultures and condors that perform these evening flights (and both vultures and condors regularly make flights like these) have been seen to perform courtship displays (such as follow flights whereby two birds fly in tight formation), so there may well be some pair bonding that is occuring during these flights, as other forms of bonding between siblings, 'friends', etc.
So, basically, it seems to provide some social bonding opportunities, it is not very costly behavior energetically speaking, and they may just enjoy it. Also, it could help them, especially young birds, master flight.