We just had the glass company come out and measure and they made the mirror to fit cutting holes for the sconces. Just make sure you don't tighten the sconce screws too tightly or you'll break the mirror.

We have the same situation as you. We have a 67 inch vanity that is fairly close to the doorway so we didn't really have space for a sconce on the side of the mirror. We consulted a bathroom lighting designer and he suggested placing the sconces (lugarno from Restoration Hardware), on top of the mirror and then place three cans in the ceiling across the vanity, thus light would come from different directions and help create more flattering light. We opted to not go with the cans, and, are just going with the sconces on the miror. We will also have a light fixture over the tub, and a can over the shower drain. For ambienece lighting we will have lighting in our linen cabinet and lighting underneath the freestanding vanity. Good luck!

Do you have end tables and lamps on each side, because that may affect the placement. Also, I would be sure the mirror is hung in the right spot first - take a good look and be sure - before you hang the sconces. The other thing to consider is the style of the sconces. I would think you might want the area from which the light eminates to hit about midpoint on the mirror sides, but that could depend on how much of the sconce falls below or above that point. Any chance of getting pix or a link to the light?

Why not buy three wood long shutters or doors from the home store, ones without ornamentation and have mirrors cut to fit? Then you can do whatever finish works best for you, after adding trim moulding around the edges of the mirror pieces,hinge them together, and you have a one-of-a-kind mirror. I am guessing there are lots of instructions on the internet for making floor screens from doors or shutters.