Replace faucet through vapor barrier

[deedubs]

I have a steam shower and so it has a vapor barrier under the tile. I need to replace a valve cartridge which controls the water volume on my shower (regular water shower, independent of steam system). These are Kohler Purist valves. When I take off the trim work, there's an opening for screws but they are empty. So it must be attached differently. I assume they didn't use screws because they would pierce the barrier. The vapor barrier seals the valve in so I can't see behind it. The barrier is clear plastic, fairly thick.

To remove the valve cartridge, I'm assuming I'll need to destroy / disturb the vapor barrier. So my question is, once the new valve cartridge is installed, how do I repair the vapor barrier. Want to maintain "perfect" seal.

Thanks in advance!