Is this the exact spot where the mirror will be hung? I'm only asking because looks like your current vanity is on the opposite wall and I want to make sure that the space you'll have for the sconces will be the same.

They should be pushed in more over the counter rather than the edge? No My lighting designer advised me to put my 6" disc LEDs centered over the edge of the counter. That way there are not shadows either on the counter as you work, or being cast onto the fronts of the lowers. Mine are spaced just about 4ft apart (center-center) and that was based on the specs for my particular lights (obtained from the manufacturer's website) and 8' ceiling, so that the "cone" or diameter of the light on the work surfaces overlapped just a bit - for even lighting. My kitchen is small, no island, so it only needed lights on two sides - the second side (with the frig and pantry, and a small cabinet with top for landing space and phone) I placed the lights away a tad more, to illuminate the cabinet interiors and center of the room more, as there isn't really a work surface on that side. I also have a small pendant over the sink, which is set in farther because I lean in farther when washing things. She also said to pay attention to whether an open cabinet upper door will block a light from illuminating the inside of the cabinet - you may have to adjust placement for that. Please do not worry about symmetry in your lighting layout! You won't be looking up at the lights to admire their symmetry, but you will notice uneven lighting!

This is the only photo I have at the moment but it shows the mirror and sink.

The important distance to consider is the distance between the center of the vanity to the center of one sink. That distance never changes regardless of the size of the mirror. The wider the mirror, the less distance you have between the sconce and the mirror. If my calculation is right based on the measurements you gave, that distance is 21" (please confirm it as I don't explicitly see it on the measurements you gave us). The distance between the edge of the sconce and the edge of the 24" mirror is 6.25". This number came from that calculation: 21" -2.75" (half the width of the sconce) - 12 (half the width of a 24" wide mirror) = 6.25". Now if you use a 36" mirror, the distance between the edge of the sconce to the edge of the mirror would be .25" ( 21" - 2.75 - 18"). You don't want a quarter in. wall between the sconce and the mirror. If you were to find a 28" wide mirror, that distance would be 4.25" ( 21" - 2.75 - 14"). I think the width of the sconce is more in proportion with a 24" mirror. I also think a 24" mirror looks nicer with a 27.5 wide cabinet below it.