Steam Overlay

I recently tried Reshade for the first time with Warhammer: End Times - Vermintide, but unfortunately my steam overlay does not seem to work despite Reshade working great. I tried installing Reshade on Player Unknowns Battlegrounds and the steam overlay worked with that. Could not find any posts here that might help.
Where do I start to solve this issue?

Alt-Tab and go to Steam itself.

Ah, come on mate, just looking for some help on what I should try. Im sure Im not the first to have this issue and there is probably a simple solution but I know nothing about how this program functions.

Both ReShade and Steam Overlay work very similar, as such they can overwrite each other in certain circumstances (so only ReShade or only Steam works). You could try a different injection method for ReShade: Instead of DLL wrapping inject the Reshade DLL manually into the process with a tool like ENBInjector. There is a tutorial on that somewhere here.