V-sync is to cap your FPS to the monitor refresh rate or in other words to sync them to avoid tearing. I should imagine you'd have to hit over 120 FPS to get tearing on a 120Hz monitor . As you would over 60FPS with a 60Hz monitor. Alhtough it's said that tearing can occure below the refresh rate anyway, but generally the consensus is that it will be less of an issue/noticeable with 120Hz.