As stated succinctly in the Tails "Known Issues" page:
Tails doesn't erase the video memory yet. When one uses Tails, then restarts the computer into another operating system, that other operating system can see what has been displayed on the screen within Tails.
Shutting down the computer completely, instead of restarting it, might allow the video memory to empty itself.
How big of an issue is this vulnerability? Does it pose a realistic security problem for users?
It also states that it completely shutting down might allow the VRAM to empty itself. Is there a way to ensure that it will definitely empty the video memory? Is this based off of the host system, or some other aspect of Tails?
If Tails is running in a virtual machine, and I exit the virtual machine (a simulated shutdown, in a sense), how does this affect the VRAM vulnerability?