It is true the laws vary by state. Try looking on the internet to see what you can find as well. I know for the state of Washington they pretty much allow employers to do what they want. Vacation and sick time are a benefit, therefore there is no law against your employer denying you ability to use it (In Washington anyway). There is however a law that protects your family rights. If you need to use sick or vacation to take care of your hubby/wife or kids I know they have to allow it. I believe that is a federal law that applies to all states, but don't quote me on that.
Good luck.