You can take screenshots using a built-in feature of SteamVR
VRto3D has a built-in screenshot hotkey Ctrl + F12 that triggers this functionality, but it doesn't always work
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