What is the etiquette on taking vacation? Should I ask? inform? or just assume and leave?

Also, is there a universal number of weeks per year that's acceptable? I seem to be averaging about 6, but I generally work half-time unless I'm on a real vacation (vs. visiting the family).

I think it depends on your funding body - I'm supposedly allowed 8 weeks per year, but don't actually take them all. I think as long as you tell your supervisor that you're going to be away, and make sure that if it's during term-time, you have teaching commitments etc covered by someone else, then it shouldn't be a problem.