You can take screenshots using a built-in feature of SteamVR
On the SteamVR Status window click the Menu Button -> Developer -> Debug Commands as seen in the image below
Scroll down to find the screenshot_request button
Unfortunately, the keyboard shortcuts won't work properly while you are playing a game and the 2D game window is in focus, only when one of the SteamVR windows is in focus will the keyboard shortcut keys work
SteamVR will open the folder Steam\steamapps\common\SteamVR\screenshots where screenshots are stored after you take a screenshot