Hi,
you cannot, because the data container of a BaseList2D is not dynamically typed (like Python itself), but bound to data types specified in its description. If you want to change the data type of an user data element, you will have to modify the description of that element via GetUserDataContainer and SetUserDataContainer. There is also no 2D-Vector type in Cinema, only the crosshair thing custom GUI which you can apply to a Vector description element.
Cheers,
zipit