I found these examples of lights overhanging the mirror. I guess it depends on whether you mind the look or not. Is the mirror purchased? If not, could you install the vanity light through the top of the mirror? It also will depend on which light you selected; some would look better than than others overhanging. Can you post your selection?

usually ok. a framed mirror might be different, but I don't think you'll notice an overhang that size.

This is the fixture I want to put. The lights would hang down about 2 inches over the mirror. The mirror is already purchased. By the way, this light fixture is 10 times more beautiful in person than it shows in the photo. Really gorgeous!

If you are still worried, could you have the mirror cut shorter by a glass company (since it's unframed)?

Go for it. I think it will be beautiful. Having one large mirror will set the lighting off nicely. Based on all the photos I've seen it doesn't appear there are any hard/fast rules. As long as it looks balanced there it will be fine.

"This is the fixture I want to put."

Look at the backs of the glass portions of the fixture that will hang over the mirror to make sure they look ok. You're going to see their reflections in the mirror.

I think there may be too much light reflection in the mirror. Different from the mirror reflecting a chandelier or sconces across the room.

Far better to overhang the mirror than to showcase the strip of wall above it.

@elcieg: "I think there may be too much light reflection in the mirror."

If the fixture uses LEDs, control them with a dimmer. Lutron LED+ dimmers are excellent.

WDCCRUISE, I'm not the OP.

You could always turn the lights up and do uplighting and that would be the cheapest and easiest way to fix it if you don't like it. I ran into the same problem. To move it up would take removing the fixture, re-wiring the electrical, re installing the fixture, drywall patch, and paint. It's laborious.