From vicarius, a Roman imperial government official who acted on behalf of the Emporer (hence the Roman claim of 'Vicar of Christ' as a papal title).
Church of England Edit
In English parochial use, a Vicar is the minister of a parish who was not formerly entitled to the Greater Tithes and would have been appointed by the Rector to act in his behalf. With the disappearance of the tithe system the distinction has become meaningless.
In modern practice the title may also be used for the senior ordained members of a Team Ministry.
Certain English Cathedrals have Vicars Choral these are the men of the choir who are not, in fact, in holy orders.
Episcopal Church, USA Edit
In ECUSA, a vicar is the tenured priest with charge over a mission congregation, that is, a congregation that is not entirely self-supporting, and which receives support from the local diocese. Vicars serve on behalf of the Diocesan Bishop, who is the rector of all missions in the diocese.