"Slaps my head around" is relative . . . just as is "excessive vibration."
Photo taken early morning, September 12th, 2007 -- I'm packed and about to leave on my L.A. - O.K.C. trip.
Note the windshield -- it is a National Cycle Street Shield, the top edge of which is between chin and nose height, depending on how I'm sitting.
I did not notice the wind slapping my head around at Interstate speeds. I also did not notice the bike being slapped around by gusts/eddies off the big rigs. This is likely 'cause I'm more tolerant of a little wind and/or compensate quicker.
When it comes to windshields you have a range of sizes from the 9x9 flyscreen to full fairings that protect the hands and well as keeping *ALL* the wind off the head/body.