How much vacation time is typical during a PhD in the United States?
I'm not aware of explicit policies regarding vacation time during a Ph.D in the US. This is likely because in the US, the Ph.D is viewed as educational, and not as a job. 

As a student, and now as a professor, vacation time was always an informal discussion between advisor and student. This is of course ripe for abuse. A job in the US usually grants 15 days of vacation (not counting weekends) per year, but in my experience that's MUCH less than what you might effectively get as a Ph.D student (in computer science) but might be comparable to what you get as a Ph.D student in the physical sciences. 

Unfortunately, I don't have personal experience with physical science programs, but via my wife and other friends I've seen that physical science programs are fairly rough on vacation. This is partly because you need to be around to tend to long-running experiments ("the flies died!") and partly because of the nature of lab work and the much more intense style of lab-based science. 

My recommendation would be, once you have some options, to ask students in the programs you're applying to, working with advisors you're interested in targeting.