I suspect that the flexible "unpaid vacation" scenario you're describing is much more rare than you think. If your team is pushing for a major release and you decide just not to show up, your days are numbered. If you spend the week working for another company, you are quite likely to be fired from the old one.
Ammon mentioned Weebly, above. They are one company that I've seen require an on-site week as a contractor. I know of a Weebly candidate who lost her current job because the employer considered the leave to be job abandonment. Thankfully, she got the job at Weebly. Perhaps Weebly's weeklong trial is effective for them at weeding out bad hires. But I suspect most good programmers would never consider giving up a week of their lives to an extended job interview.
No vacation days burned. No money lost.