want to change out old sink but not replace entire vanity top
If it's a drop in sink, you should be able to remove it and replace with a new one. You need to disconnect all the plumbing, faucets, etc. and the break the line of caulk around the outside. It should lift out of the vanity top.
Good luck with your project, Valerie. I hope you'll share it with us when you're done!
you would need a sink the same size - here are ideas for you