In nearly every case (California is always special, as are a few other states), you aren't required to have a vacation policy at all, or any company policy for that matter. One of the main reasons you have company policies is to make administration easier, and more importantly, to protect you from legal troubles that result from discrimination. If you have a clear policy about vacation days and how they're paid out, and you adhere to it, it will be much harder for someone to claim it as evidence of discrimination than if you sort things out on a case-by-case basis, without any sort of policy as a guide.

Outsourcing partners will require it for similar reasons, but also because it makes their lives a lot easier when they handle it for you.