We are running projections to see the fixed income from leasing the lobby (minus popcorn and soda, we woudl still run that) or running the space ours self is more profitable.
Well, while you're running your projections, consider this:
A customer is only going to spend so much on concessions items, no matter how varied your menu is. That dollar can pay for the item, the staff, the vendor and you, or it can pay for the item, the staff and you. Seems an easy choice to me, but if you have a different take on why vendors might be a good idea, I'd like to hear it.
While I understand there are, or were companies that would run your concessions, or equip it for a part of the take, I know of no such operation currently participating in any theatre in my state. As such, I can't see how it would be a very popular option.