When vultures congregate like this the food has been spotted. Some carcass is being eaten by lion or other carnivores close by. The wisened vulture would be ill-advised to drop in on that feast. Waiting one’s turn is a life rule that in this environment prolongs life.
The king of the jungle is known to go for a little nap after a good session, before resuming its efforts at devouring everything. That would be the time for a quick foray, choosing the moment prudently and relying on vigilance plus strong wings for the getaway.
Or, what also happens is that some animal has just dropped dead somewhere, instantly transformed to food for those that cant hunt. You can’t afford to be too fussy if you’re in the scavenger business.
Vultures have been feeding in this way for aeons, mindful of the dangers like hyena and what not.