As per the EJB3 spec, Vector is not supported collection type for OneToMany mapping:
Collection-valued persistent fields and properties must be defined in terms of one of the following collection-valued interfaces regardless of whether the entity class otherwise adheres to the JavaBeans method conventions noted above and whether field or property-based access is used:
java.util.Collection, java.util.Set, java.util.List, java.util.Map.
For collection-valued persistent properties, type T must be one of these collection interface types in the method signatures above. Generic variants of these collection types may also be used (for example, Set<Order>).
Vector shouldn't be used in any code written past circa 1999 anyway, except for code that needs to talk to legacy classes like some Swing components (and even then, I'd sooner implement my own subclasses than use the Vector based ones).