I took out the old vanity today. What was left behind didn't appear to be too bad. I cleaned it out and touched up some paint, since it is easier with nothing in the way. I installed a more open style of vanity, to keep an eye on the situation.
Something similar to this, but not nearly as expensive or 'fancy'. That kind of open though: Pegasus Element 24 In. Espresso Vanity - BWV-017-24 at The Home Depot
If it comes back, I am going to rip out the drywall. I hope it doesn't get to that point.