What size vessel sink suggested for 18 inch vanity? Should I not go larger than 14 inch? Thanks all!

go to the box store with a piece of cardboard, put it on top of your favorite sink, draw the circle from underneath with a pencil. step back and cut it out.

now put that on several vanities, and see how it works out. you have to have enough vanity around the vessel so you are not likely to break it with a mop, moving stuff into the bathroom, flailing on the slippery floor, etc. ideally there needs to be room for personal products and stuff on top as well.

if it's too close for the pill bottles, curler, hair spray, toothpaste, soap, et al, you don't have enough vanity space for that vessel.