I can't speak to the policies of generic American firms, but I can speak to your negotiating power. If you have multiple offers, and the sticking point for you is their vacation policy, do make that clear. It might shake loose an exception to the supposedly hard-and-fast rule.

(Or, just become an academic. There's no official vacation policy, but then nobody really cares when or how long you take off as long as you're around enough to handle your deadlines and other responsibilities. Going to a conference in an exotic location during the summer? Stay for an extra week? Sure, why not? One of my colleagues spent most of the summer in Europe. Of course, he has tenure...)