Install "old work" round boxes where you want the lights to be. Run the existing wiring to one of those boxes instead - (the existing wire had better reach one of them!), since you're you're going to be eliminating/patching the original box. To get the wire to reach, you may have to deal with loosening a wire that's secured (stapled) in the wall to the stud. Pull addl. wire from the first box to the second box (drill stud carefully). Repair wall, install lights. If you're dealing with older style wiring, or you feel you're in over your head, hire a professional. Consider it an investment in safety.

Lots of replies! In our master bath we have a rectangular mirror with a pretty frame. Then because the space is only about 4 ft. wide with a wall on both sides, I had the workers put an inset medicine cabinet on each side. When I want to see the sides or back of my hair I just open the appropriate side mirror. And we each have our own medicine cabinet! Works for me. Plus it reflects lots of light from the sconces on each side of the framed mirror. I also had them tile up that area under the framed mirror about a foot high. Keeps lots of water splashes off the mirror. Looks nicer too because the tile has a beautiful 3 inch high band of "deco" tile that continues around the 3 sides of the area. There is just one sink in the counter area because my husband and I are hardly ever needing the sink at the same time. We separated the toilet, tub, shower part of the bath from the sink with a pocket door so that while one is in there, the other can be using the sink without a steamed up mirror and all. It works great. I had seen this set-up in some motels and liked it a lot. Makes the bathroom do double duty!

I would use the full mirror. I can picture your kids being teens (if girls) and having a sleep-over where three girls could get ready with a full mirror. Though my son as a teen could spend a lot of time fussing with his hair, so maybe it's not so gender-specific. Pottery Barn has large framed mirrors like the one in your first photo for a few hundred dollars. I know they have 60", not sure about larger. But I'd get a frameless beveled mirror from a glass shop. They can cut holes to mount sconces, and I think it's a good idea to use three because you have a wide space between sinks with that style vanity. You might consider bar style sconces so that you can put them on both ends and in the middle. They're 4-5" wide and won't impede the view of someone at the sink. Side sconces provide the best lighting--no shadows fall on the face from overhead lights. Mounting lights on the mirror will also bounce the light, nice to have in a windowless room. A new look for full-length mirrors is a short mirror about 30-32" high, framed or unframed. I love this look for a long vanity. Less mirror, less cost from the glass shop!

A double light over each sink should be sufficient with a window and assuming the shower is directly across from the vanity and that is the extent of the room. Other options are three single sconces mounted on the mirror and an over head light to give more light to the shower. I have a four-light over a 60" vanity in a 5x11 windowless room. The vanity area is well-lit. The shower at the end of the room is a little more shadowed but not enough to make showering and cleaning an issue.