Use two single vanities that are installed in such away that there is ample sitting space between them. Then use one large counter top piece to bridge them together.

Thought of that but wanted to see if someone had seen one.

Found this from ronbow, but would be limited to small stool that can be pulled out. Think zandilen's idea is best alternative

Sorry, but the cabinet above would not give you a vanity ...too narrow and too tall. Sink vanity height now a days is 36" high, vanity should be at 30" high. Width should be a minimum of 24" to 36". Using two vanities and than lower the counter to 30" off the floor with a drawer or just a front panel. Or a long run with a vanity counter between the end wall and cabinet. Cabinet maker can do anything.