To get a true comparison between countries, you also need to factor in official holidays. We have quite a few in the US, a few more than most other countries. And we contrive to make as many as possible 3-day weekends. We probably do tend to get more long weekends in the US than in many other countries, which tend to take their holiday in one big chunk. I'm not sure but what shorter, more frequent breaks might actually do more good for body & soul with less pile-up and mess back at the office.
Tourism is a major industry in the US, and it would definitely benefit from an increase in vacation days.
There is much to be said for companies and institutions requiring anyone in a position of authority to get out of the office for at least an entire two-week annual time block. It frequently happens that those with something to cover up do so by never, ever going on vacation.
Kind of like the Laffer Curve, there has to be a productivity curve. If you work zero hours a year, your productivity is zero. If you "work" 365 days a year for 24 hours, your productivity is zero because you will be in an insane asylum. The magic is finding the right number. How hard can you work, until your productivity declines?
We need to take play as seriously as work. The problem as I see it, and as others have mentioned is that many people don't really get a true vacation. A true vacation means no umbillical cord to the office. You are a free agent, free to take a holiday from work. I would suspect that the days a worker needs for vacation to be productive, would actually drop if middle management types actually allowed people to not do paperwork and return phone calls, while they are enjoying their holiday time.