A Sub-Dean is a priest or minister who acts as an Assistant to a Dean either in Church circuit or an academic institution. He or she is however not a Vice-Dean. A Vice Dean is a person who can deputize a Dean whereas a Sub Dean can only assist or carry out assignments as may be delegated to him or her by a Dean. A Vice Dean can exercise the powers of a Dean in his or her absence but a Sub Dean only report to a Dean and does not have the power of a Dean.
References
Sub-Dean Wikipedia(Text) CC BY-SA