One vendor stated that the floors would need to be leveled prior to installation and that all concrete floors had to be leveled prior to glueing down the floors. Is this true?
It's true if your floors aren't level enough.
The flooring manufacturer usually has a subfloor level specification (e.g. 1/4-in over 10 ft or 1/8-in over 6 ft, etc.). For engineered flooring in a glue-down installation, I would think that the level specs would be pretty tight. Otherwise, the joints could fail.
If I were given two quotes for a job such as this, and one guy looked hard at the floor and said he'd level it first, that would be the person I'd go with. As a customer, you don't want call backs any more than the installer does.