The Vector class was one of the earliest of collections available in
Java, released with Java 1.0. The ArrayList class was released as part of the Java Collection Framework, some time later -- I am thinking with Java 1.2.
Some work has been done with Vector to integrate it with the framework, such as having it implement List, and support Iterator, but it also must support the old API, for backward compatibility purposes. As such, it doesn't seem to fit exactly with the framework -- and also seems to be duplicated by other collections in the framework.
Henry