THAT is a good question, I believe Billy, I have often wondered the same myself.
From what I can tell the respective titles seem to have specific nuances for specific sects of the Christian church. The modern titles are in some cases derivatives of the word's origin as well.
For example, REVERAND is an honorific title given to ordained clergy in most Christian churches.
PARSON is from the Latin word persona, meaning "person." From the eleventh century English, where there term was a legal one, applying to the parish priest, because in all matters he was the designated "person" to deal with. Today, the term is not used as often as it was, and often evokes rural connotations.
PRIEST on the other hand is a special term for an ordained minister of a Roman Catholic or Episcopal or Orthodox church.
In Roman circles, the term refers to those who recite the Mass, but the Episcopal Church traces the word's origin to a Celtic corruption of the official term for Clergy - Presbyters.
VICAR is from the Latin word vicarius, meaning "a substitute." An English term referring to a priest in charge of a mission. Technically, the diocesan bishop is the rector of all diocesan missions, and vicars are appointed to their mission by the local diocesan bishop to represent him or her.
The term "Vicar" is still the terminology used today to describe an English priest in who is charge of a congregation.
Personally, I prefer man/woman of the cloth as a convenient catch-all!
http://www.tonyhj.ca/Priest/glossary_of_titles.html