My favorite look for a long vanity is a narrower mirror. I like the proportions of the two, and you have just the right space to insert a narrow mirror. You could use a flat 2" white or walnut frame or go frameless. That would be my first choice. If you prefer frameless, on an open wall like yours, I prefer beveled frameless mirrors for a more finished look, but that leans a little more traditional.
If your backsplash is tall, you'd have to rest the mirror on the backsplash. If you only extend the mirror to the sink edges, you may get away with not having to move your outlets, or you may have to move them only a little. Mock it up to see how a mirror would fit. Then use contemporary bar sconces over the mirror. I wonder if they used three smaller sconces here, which lights the middle of the counter too.
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