I printed a bunch of vintage 11x14 glass plate negs a while ago. Surprisingly a couple negs had flatness issues and I was unable to print them sharp, as I wasn't using a vacume. (however you spell vacume) I was just clamping the negs down to the paper on top of another piece of plate glass that I know is flat. I was unable to get a couple of the prints sharp in areas regardless of how much I clamped it, the glass was just too warped.

In Rolleiflex lore the camera was created when Kodak figured out how to put film on rolls but it wasn't good enough for a lot of photographers. So Rollei created a plate glass back as an accessory. If anyone cares.