All those are good points, but if your quad is always yawing to the left i dont think its a vibration issue, or a PID issue as that would produce random issues. It is more likely to be an ESC or motor issue. When I had poor Yaw PID values the quadcopter would yaw left and right randomly.
If you go to the motor tab, and test the motors (make sure to remove the props first) you can plaay with the trottle values to see if any specific motor acts strange, like one might make a noise (bad bearing) or it might spin faster/slower than others.
That should atleast allow you to try pinpoint the problem.
Another thing to check, but unlikely the isssue is to make sure the RC inputs are centered, particularly for your yaw input.