Well, let’s start with the conclusion: setting
XV_SYNC_TO_VBLANK has no effect, if a second monitor is attached.
But let’s start at the very beginning: What is tearing? Well, this is how Wikipedia defines it:
Screen tearing is a phenomenon in video where a previously rendered frame overlaps a newly rendered frame, creating a torn look as two parts of an object (such as a wall) don’t line up…