Sorry, this may seem rediculous, but is your windows activated or are you in notification mode? In my experience, both a friend and my self have had this issue when in notification mode. (EXTREME like never seen before tearing.) I spent many hours in the past and found almost no information regarding it. If everything is fine there, never mind.
Other thing coming to mind, if you have an NVIDIA card, go into NVIDIA settings and try resetting the defaults for the global profile, then try switching VSYNC on and then setting it to adaptive mode in the global profile. Try that and see what happens. That is how I figgured out way back when that it was windows notification for activation, the problem was so bad that even the intro video for FNV was tearing, EVERYTHING was.
Setting VSYNC via driver panel in FNV alone didn't do anything for me, it was global, which, like I said, told me it was another application causing the problem. (yes, even with steam overlay disabled.) If this doesn't apply to you, sorry. I hope it will help.
Also, make sure your power saving feature is set to prefer maximum performance in the nvidia panel, disable link state management under the windows power settings, also make sure your CPU cores aren't getting throttled or parked by temps and set them to 100% minimum.