An optional compiling tool that Calculates the lighting of a map, It gives parts of the level, called patches, a certain amount of brightness depending on how much light it would receive. This is the slowest of compile tools due to the many, many calculations it must do to acheive this.
Side note: if you are watching the compile dialog and it looks like it has Hung on about
9... this is because Vrad takes longer with each light bounce calculation. Be patient.
Win NT/2k/XP/2k3: Also it is possible to open "windows task manager" (ctrl+shift+esc) and in the processes tab find "vrad.exe" and set it’s priority higher. (exercise extreme caution fiddling with these settings.)