I can't imagine how, since "vacation" just means being paid for not working.

Buying vacation would just mean not being paid for that time.

Here in California, vacation/PTO is treated just like wages for regulatory purposes, including final paycheck.

What I can imagine being hard to buy is company holidays, but I'd rather have the "vacation" (read: cash) instead.

In your experience, is it actually as easy to get unpaid time off as you make it sound? Certainly, not being paid for not working is entirely possible, but it's much less desirable if you lose your job for it...

(My job allows you to "buy" extra time off, so it's not an issue for me; but I figured this was uncommon, especially in the States.)

If I make it sound as easy as taking paid vacation, then yes, but with a small sample size.

The much larger sample size I have for a related question: is it any easier to take vacation at a company which offers 3 weeks instead of 2 weeks annually? To this, I've found the answer is "no."

Both situations have a simple explanation, which is that, except for the smallest companies, the person(s) setting the PTO policy is not the same as the one approving absences. That is, my manager cares about my availability, whereas HR and/or Accounting cares about how much I get paid during such absences.