I think XF and VSF is the same factory changing from V6F, KW.
All Tudor models listed as XF factory. Some pams (mostly special pams) such as PAM438 & 441 and PAM 531 (use their GMT movement with same hour hand adjustable and use case material like gen), PAM382 Bronzo (use their 9000 mod. movement with correct balance wheel position) and PAM616 Carbotech (with same material like gen.) all listed as VSF factory.
So I think now they use XF for standard models and use VSF for the special models that they are better than another factory rep.