A recent employer gave me 3 weeks of vacation. Great! Never mind that they kept asking me to delay it, for a year because each artificial deadline they created ended up slipping. There's never a good time to take vacation.
EDIT Therefore, it's always a good time to take vacation (given sufficient notice).
There's nothing difficult about implementing it. Clearly your employer just doesn't buy the concept at all. They probably only offer 3 weeks (slightly higher than average) as a recruiting incentive, which is pretty dishonest if they don't actually let you use it.
The reasoning behind this concept is what's important. It's about treating employees as equal partners in a relationship, as opposed to children, like most employers do.
Definitely read this essay if you havent: http://paulgraham.com/opensource.html