Peldi, I think you're trying to do the right thing by treating people with respect and allowing them the freedom to make their own choices, BUT I think you are

Peldi, I think you're trying to do the right thing by treating people with respect and allowing them the freedom to make their own choices, BUT I think you are ignoring simple human nature and actually making their vacation time worse for them. As others have said in the comments, in systems like this, people will take less vacation than if there were a defined plan. Because there is no defined plan, one is set by example. They look to you and to their peers. I suspect strongly that you will take less vacation than them being the CEO of the company. I also suspect that their peers will gravitate toward taking less vacation, and human guilt will be the guide. I urge you to reconsider. Instead give them a very generous vacation policy (20 days! a whole month!) and then MAKE THEM TAKE IT! You can do that by simply telling them 'You need to take a vacation' or just have a max cap on vacation (say 40 days) or have vacation expire after 2 years (time enough to build up a nice sabbatical).