Actually, v-sync is always on in our versions of the STALKER games.

During tests we got about 2500 - 4000 FPS in the menus, and there were reports of all sorts of unpleasant consequences (from squeal coming from the graphic card, through interference on the sound card audible on the speakers, to overheating of the hardware), so we decided it's safer to force v-sync.

V-sync is forced by modified game renderer files - you can ask GOG Support, and they'll send you the original files.