I would choose one frameless mirror to fill the space above the vanity.

If you want sconces they can be mounted on the mirror, but a long overhead fixture would be neat and clean. You'll now get nice natural light from the window above, congratulations.

Here are examples of transom windows over vanities. You can place the sconces to the sides or directly above a mirror.

One long mirror with long sleek vanity light above mirror will look better imo as the transom window has the arch and grills so you dont want to go with clean lines underneath

One long mirror no sconces since those will just advertise the window is not centered and do one nice long bath bar above the mirror

I’d hold off on the electrical placement and mirror decision as long as you can, until you have a better sense. The window will come into play regarding balancing that wall as well. In the meantime, tape out ideas on wall and/or do diagram and to help visual and decide. The ideal if budget allowed would be to reconfigure the window. Perhaps a quote could help you decide, as you may regret down the road.

Two rounded rectangle mirrors for two sinks. Three cylinder sconces for side lighting. Overhead lighting is aging. I would avoid misaligning the window and vanity (just wrong looking). I would keep the vanity size as current. A larger shower would be a luxury alternative to fill the space. A wall between the shower and vanity would be another good alternative. You can make that decision easier by deciding about wall tile start and end locations.

Having very limited information, my thought is to create a beautiful feature wall - tile the entire wall to the ceiling, add glass for the shower divider to visually expand the space, two mirrors over the sinks, a double sconce in between and layered lighting for support.

What is the lighting plan? Will you have ceiling lights, a light above the shower, a statement lighting piece? You'll gain daylight by removing the bulkhead but the vanity lights can't carry the entire space. What about the wall behind the vanity, will there still be closet doors and open storage? Whatever is on that wall will be what is featured in one long mirror.

Layered lighting on dimmers in the bathroom help transform usage and aesthetics.

Create a rough drawing showing the elevation - cabinets, sinks, window, countertop, outlets, tile plan and new shower divider, then back into options for the mirror(s) and light placements. You can mitigate centering by carefully selecting mirror size over the sinks, and adding a large, beautiful ceiling mount fixture can help balance visual weight overhead.

So I’ll add some more info. The window cannot be changed (not in budget). Also, the vanity is already ordered and it’s custom so cannot change the size. The reason for a wider vanity is storage. The new vanity will go from full wall (right) to a newly built half wall (left) and the new wall will have glass on top as a shower wall (see inspiration pic). The ceiling is vaulted (see pic). Beyond the tiled shower, all walls will be painted.

There's not enough information in your bath; any possibility of flipping the vanity to the tub area? The shower in place of vanity. The window will always look off in relation to the mirrors and vanity. Your bath dimensions would help.

Vertical sconces slightly above eye level are best for makeup, if that matters.

If you’re using two mirrors, get three sconces. If you are using one mirror, use the two on the sides.