I do not like sconce lighting in bathrooms so why not look at a nice wide bath bar type light and IMO a wall of mirror instead of that tiny one

A..........Irrelevant:

"I do not like sconce lighting in bathrooms"

B....... Show a picture

Math: 42 INCHES minus a mirror 24 inches wide, leaves a total of 18 inches, to be divided by the two sides......leaving 9" for each in total width.

C There are many that will fit......

D No way to advise beyond that, as you have no pictures of the vanity, which wall, left or right, what is next to it, or even if this is a full bath or a powder room. We've no clue your style, what sconce you may be looking at.....

Items B, C, D are all relevant.

If the vanity is boxed in on one or both sides, you're right, nine inches is tight for sconces. Have you chosen sconces? I know Patricia said she doesn't like sconces, but unless it's a powder room, I strongly prefer them. Since it's your bathroom I think you need to do what you prefer. If you really want sconces I think it's worth getting a different mirror to make it work.

A 42 inch vanity, with a 30" diameter mirror and a light fixture above, also works.

Show what you have.....

Jan's advice is spot on. You could also do a framed mirror that goes the full length of the vanity and have cut-outs in the mirror for sconces. Like these:

Beauty and the Baths: Primary Bath · More Info

Beauty and the Baths: Guest Bath · More Info

I like this option because the mirror is large and makes the room feel more open.

42" vanity, 36" round mirror with articulated lights mounted above the mirror.

The sconces will be 4.5 inch diameter so that will leave about 2 inches on each side of each sconces. I wish I would have gotten a 20inch diameter mirror instead of a 24 inch. It was a very expensive mirror and I am outside of the return window so I think I’ll just keep it, get small sconces (4.5 inches width) and cross my fingers

It's hard to make a better suggestion, when you are reluctant to show ALL the elements for the bath.

There's no OTHER place in your home for a 24 x 36 mirror? What do the sconce, the vanity and the mirror look like? : )

I can tell you the most easily sourced will be 18 x 36, you won't miss two or four inches and they ABOUND...........whatever the finish

Now you have a foot on both sides....with comfort zone to boot.

The 24” x 36” mirror, centered over the vanity will allow enough room for a sconce that is fairly narrow. We just finished our bathroom project and our vanity is 84” (twice the size of yours) and we put sconces on each side of the mirrors. Look at this picture and you’ll get an idea of how yours could look.

Are Those mirrors are 23x36”? This looks great! We will have 9 inches on each side of mirror and our sconces are 4.5inches wide