You might want to try a flip up helmet
http://motorcyclehelmets.net/yvs500/shopexd.asp?id=31I have the above helmet, and it works very well with glasses. The price is right too.
Perhaps it's just me, but bifocals cause me problems when looking at the mirrors at night. It took me x hundred feet of road each time to figure out what I was looking at. I sent my Wiley prescription glasses back and had single vision lenses put in.
