Get a narrow feather duster!

The only thing I can think of right now, is to frame out the 5.5" top and both sides. Top and face the front of the spaces with the back splash wall tile. Frame the cabinet with this extension, level and flush to existing top.

Or ... face the extensions with the vanity top marble (if you have some left)

Another thought if you do not have enough material left from the wall tile or marble, you can use the marble on the top framework only, leaving 2 long narrow open (narrow framed) cubbies. Add shelves if possible. Store rolled up towels in the openings. If possible I would face the front edge of the shelves with the marble trim. Or paint shelves the cabinet color.

Drawers and cubbies in a lovely bathroom · More Info

Is the vanity centered?

Paint the inside of the side walls to make them part of the vanity area- by lowerimg the contrast. . -

(paint to coordinate the mirror wall,to lessen the contrast. (See below)

Allow the niche to enfold the vanity, like wrapping arms around it.

Towel rail each side ...or just leave as is

I agree, leave it alone, looks fine. You start trying to fill that in and it’s gonna start looking ridiculous. I like tour selections, very pretty. What will you be doing for a mirror and lighting?

You purchased a free standing vanity. It is not supposed to reach the walls because the top hangs over the base cabinet is set back. If you wanted it built in you should have purchase vanity components for a built in vanity.

The time to “do something” was in the design phase. That’s when you should have gone for the built in vanity with the custom countertop that filled the entire space. It’s too late to fix that now. Live with what is.

I changed my mind, I agree to leave as is. I noticed the legs, therefore the best approach is to keep it as it was intended. I think you were just comparing it to the custom built-in units. Maybe you were thinking it was wrong.... it is not, it's perfectly fine. It's up to you what you prefer.