Have you worked at a company that offers unlimited vacation?
I do (it's a recent change for us). It works fine. People take time off when they want to (one of the guys on my team is on a 2-week vacation in Thailand right now). Yes, some people (including myself) do the illogical guilt thing on occasion. But if your manager isn't encouraging you to take time off periodically, he's doing a bad job as your manager.
Right now I'm planning a week and a half in Peru for this spring, and I'm toying with the idea of a southeast Asia trip this summer for a similar amount of time. I also took four days off beyond normal holiday days around the New Year. A week and a half ago I was very ill and stayed home for three days without having to worry about "taking a sick day" (not to mention that my colleagues thanked me for not coming in and getting them sick too).
If you work for a company that has "unlimited vacation" but makes people who use it feel stigmatized, quit and find a company that doesn't lie to you about its culture. They do exist.
It makes me sad that this "unlimited vacation doesn't work" comment is the top-voted one right now. It's absolutist and entirely nonsensical.