I would recommend using a 30-inch-wide mirror. That would leave you 9 inches on either side to hang your sconces. The sconces would be centered between the edge of the mirror frame and the end of the vanity. If you wanted to mount the fixtures right on the mirror, then I would recommend using a mirror that is the same width as the vanity (48 inches). You will first have the electrician install the junction boxes for the sconces in the wall. Then the mirror installer would come and measure will exactly where the junction boxes are and pre-cut the holes before installing the mirror. You can add the framing for the mirror after the mirror has been installed.
The vanity lights should be mounted at 5 feet 6 inches above the finished floor. What you want is to have the translucent part of the light fixture at eye level. If you live with somebody who is particularly taller or shorter than you, then I would recommend getting a long vertical fixture so that both of you will have even illumination on your faces.