Vibrations will remain an issue unless you are on a solid concrete floor. Especially with six mirrors, any change in angle by vibration is amplified and the end point of your laser will be all over the place.
A possible solution to this would be to use slightly divergent (or just a much wider) beam, and/or an array of sensors on the other end so that when the beam is a little off it simply activates another sensor.
A possible solution to interference is using modulated IR.
Look at how laser tag games work: they have a red beam that shows you where you're aiming, and an invisible IR beam right next to it that does the real work.