Help! I bought a 30" vanity that has a rail for nailing that runs exactly over my hot and cold turn offs.

I had the same problem once. I had to remove most of the nailing strip to clear my cutoffs. Not knowing how yours is constructed this may not work for you but it did for me. I made a couple of "L" wood brackets about 20 inches long and used a good glue and some small screws to fasten them vertically to the inner sides of the cabinet, where it touches the wall. Be careful that the screws don't come through the outer wall of your cabinet. After the glue dried I drilled holes in the bracket and used anchors into the wall to hold it in place. Depending on your wall you may find a studs to anchor to but in my case I used erfly anchors into the drywall. It has held for years. Good luck.