the Larousse states that the viguier is an old Provecal term, meaning a court magistrate who dispensed justicve in the Midi region prior to 1789.

from Latin vicarius
a very old LaRousse tells me:
magistrat chargé d'administrrer la justice au nom des comtes ou du roi dans certaines provinces du midi de la France, avant 1789

That of course does not mean that "vicar" is the correct translation but the above reference might help you along.

By the period you're talking about (16th century), Mary, the essence of the term was that the fellow was a *royal* magistrate.
The word does, indeed derive from the Latin _vicarius_, but that word went through considerable permutations in the course of the middle ages and in modern times --the English "vicar" doesn't have any of the connotations that the Latin originally did and most definitely should not be used.
"Magistrate" may need some qualification, as well.
Before the 13th century, when the Midi largely came under Royal domination, the _viguier_ was an officer of the Count; after the Kings' takeover of the region he became a King's officer.
Cf. the CNRS' _Trésor de la Langue Francaise_ :
"Juge qui, a l'instar des prévo^ts royaux dans les autres provinces de France, rendait la justice dans le Midi au nom des comtes, puis du Roi....prévo^t royal....viguiers royaux."
Simple "magistrate" may not be enough:
it depends upon how techinical the piece you are translating is, and to what audience it is intended.

Always best to not jump to a quick judgement in this Kudoz forum, btw.