I agree with Mark Livingood about the Take-a-look mirror. I also have mine attached to my helmet, using the little clear plastic adaptor plate that they supply. This permits the mirror to come off when it bumps anything, but that is how it should be, in my opinion. It is very easy to put back on. By the way, I also use prescription sunglasses that have progressive lenses and I have no problems with the mirror. I position the mirror in the distance vision portion of my lens.
My wife (stoker) also uses a mirror, since she is so used to it from her single bike. She uses a third eye mirror attached to the temple of her glasses (also prescription - bifocals). This works as well as a helmet mounted mirror, since either type of mirror changes its view as you rotate your head. The third eye mirrors use a ball and socket arrangement to aim the mirror. In my experience using these, the socket usually loosens after a while, necessitating a replacement. The Take-a-looks seem to be a more robust design from that standpoint.
Like Mark, I feel naked riding without a mirror any more, after using one for the last 20 years of cycling. My wife feels the same and it is not a big deal for both of us to have a view of the rear. She sometimes alerts me to something coming up from behind, although most of the time I'm already aware of the situation before she tells me. Not a big deal though. I would recommend these over a handlebar mounted mirror, but whatever works for you is the way to go.