Very hard if even possible to fix warped drawers. Is it the fronts? You can replace those.
If they're particle board which is used in some cabinet construction. If that's the problem, they can't be fixed. And the same is true if it's wood and it has warped. Either way, you'll need to get new drawers if the rest of the vanity is ok. You may have to replace the whole thing. I would suggest that you get and use a good exhaust fan that is the proper size for your bathroom and then run it a little before, during and a little while after someone showers or takes a bath.