I've never sent a letter about vacation. Now that Ali is a little older, I try to avoid missing as many school days as possible, but we did trips during the school year when she was younger. Our school doesn't have a mechanism for an excused absence for a vacation, but we've never had a teacher who didn't allow makeup work. I look forward to chatting with the truancy officer after each trip or several small illnesses. She is required to call after 5 unexcused absences, and if you are going to pull the kids out for vacation, you might as well make it a full week, right?
Cute related story - in 3rd grade, Alison's teacher said that all she wanted was for Ali to keep a daily journal of her trip to work on her writing. I put together a book with all kinds of activities (daily writing, daily spending calculations, pages for travel, people we met, license plates we saw, etc.). When she turned it in, her teacher took it around and showed all of the other teachers, not because it was so great, but because she had never had a child actually turn in the requested journal before.