I would do square as you need nothing more going on, too much already.

Best to show an elevation of the vanity wall…is there just one vanity or two? What are the sizes of the mirrors and how is the lighting being planned (sconces flanking the mirror/s or over head)? Doing a sketch by hand is fine as long as you show all the dimensions including ceiling height.

And, don’t forget to consider faucet height and door swing clearance over it.

Sorry - one more ‘and’…also show what the medicine cabinet for the side wall - what that looks like and the mounting height need to be carefully considered with the vanity wall items.

As other posters have said - there is a lot going on in your bathroom already. Adding a new geometry like a large round medicine cabinet might push things over the top.

My SIL stuck a round medicine cabinet in her house for sale. It looks fine, but because of the shape and the layout of the shelves, it really holds very little. Be sure to check the inside to see if it suits your needs.

Go for it ! We think a circle or pill shaped mirror would be a great addition to your space.

or go a little more subtle with rounded edge square shaped

Round mirror softens the area and is also better Feng Shui, no sharp edges.