Vapor is another word for moisture. A vapor barrier is a sheet of plastic covering the entire surface under the floor to prevent moisture from causing the floor to warp or absorb stains. You would typically use a vapor barrier in basements with concrete floors.
Yes, you can install a laminate floor system over existing hardwood floors. Just be sure that the existing hardwood is pretty smooth or the hump in the floor will transfer through. You must use a vapor barrier over your existing hardwood to reduce sound as well as moister transfere between two floor systems. Be sure to let your new floor acclimate to your room temp. for at least 48 hrs. this is especially important if you are installing floor in the winter.
Friction will stop it.
Nope, not good fro the wood and really not good for the finish. Wood LOVES moisture but moisture damages wood, so you should really be cleaning your wood floor with very minor amounts of an oily based cleaner, NOT with anything with water or steam.
If a wood floor has been properly varnished, it will be waterproof, and you can wash it as you would any other floor. If it is just a matter of removing dust, you could use a Swiffer.
No