The census bureau charges $60 per cd, but once you get that cd you are fully authorized to redistribute it, that's how it can be obtained for free.
Or you can download it for free, they hide this pretty well:
I'm not sure if they are vector based or not. All I know is that they need a considerable amount of conversion before the data can be of any real use. Also, the maps tend to be less accurate that what you are probably used to.