How will you keep yourself current with your cabin crew training until that time without doing any flying? You wouldn't want to risk passengers lives with your own lack of knowledge and experience would you?
Currency for VCC is the same as for CC, and the limits are set by the CAA, who are happy with the procedures put in place by BA to address this issue. BA will ensure that VCC are current or they will be dealt with by the CAA in the same way that they ensure that CC remain current.