The end of the calendar year is a crazy time for many businesses and the rush is on to fill orders, finish transactions and get out of the office for some much needed time off. This obviously applies to your employees as much as it does for you. You need to ask yourself if you resemble Ebenezer Scrooge in your attitude towards taking time off...
Are you a do as I say, but not as I do manager? You tell your folks about the importance of taking time off, but you never do it. You encourage them to take off, and are great about letting them go, but you can't pry yourself out of the office for fear that something will fall apart while you are gone. You are not doing your employees a favor by doing this. Your actions speak louder than your words and many of your employees will feel that the unspoken rule is to not take vacation, even though it is offered. The fear of what you will have piled up for them when they return may make many of them rethink their plans. You need to practice what you preach.
Or, are you a do as I do, but not as I say manager? Do you talk a good game about your employees taking time off, but never really let them do it? This is the worst type of person to work for. You have a pretty employee handbook or policy that tells them how much time off they are due every year, but when they try to take it, you block the dates, or worse let them take it off but keep them tethered to the office by the phone or email.
Vacation time is something that you and your employee agreed to at the time they were hired. It is part of their benefits of working for you that you agreed to upfront (generally), and it is not something that they have to earn every year, they are entitled to it based on the policy you have in place. That being the case, breaking that agreement by making it difficult for them, or dissuading them from taking their days off shows a real lack of integrity on your part. Plan ahead and come up with a schedule that works for everyone. If you do have to limit when people take vacation, tell them at the beginning of the year, not right before the holidays. Start scheduling time off at the start of the year, including your own, and everyone will benefit from it.