Can V-Sync avoid tearing? I think it can't. Am I wrong?
V-sync means that the GPU won't switch buffer until the current frame is done showing. 

When you tell to swap the buffer with V-Sync enabled the actual swap doesn't happen until V-Blank, a small window between frames where nothing is being sent to the screen.

The reason I think you cannot see the update tear is that it moves across the screen during the update. While a non v-sync tear stays in the same place on the screen for 16 milliseconds.