BT vision boxes dont use cards so dunno why yours has one inside.
BT use a cardless certificate based system which, to all intents and purposes, is uncrackable.
The boxes themselves can, however, be played with to allow them to be used as pvr boxes on freeview if you have sufficient technical knowledge. Afaik, there is no simple "readme" on doing this - you actually need to know how to get your hands dirty and do some real coding !