In which cases should I use the "Use VRAM" option in the Preferences?

You should try this option if the display is slow for an unknown reason and/or if you experience display refresh problems. Note: if you distribute a stand-alone application built with this option, and a user reports a display problem, tell him/her to use the /NOVR command line option to disable this option on his/her machine (create a shortcut to your application, edit its properties and add /NOVR to the command line).