Lambs are pretty helpless. They can't do much about corvids. Chickens run and flap and squawk, they're a lot of effort to get at. Either way, corvids predating on things their size or larger is fairly rare, it's too much effort. If you see an entire mob of them watching your chickens, you might have cause for concern, but one flying overhead is no problem.
Vultures are scavengers, unless driven to be otherwise. In areas with a lot of farming, where all the dead animals are farm animals that are quickly disposed of, they don't have enough food and will resort to attacking live prey. They really aren't made for it, though- their beak and claws are weak. Black vultures in particular are the ones that more easily turn predator, and are known to kill weakened small animals, but unless they're especially hungry they won't go after a healthy chicken. They're smallish vultures with black feathers and greyish-black heads, and are often seen in groups. Turkey vultures are usually solitary unless gathered around a kill, are larger and have a hint of brown coloration, and have pink heads, and they're a lot less aggressive in all circumstances.
