where is a dog coller (for Vicars, Priests, Ministers etc) so called?

Answer 1

It is called a clerical collar.

A Roman collar is a style of clerical collar and the wearer is not necessarily a Catholic.

Clerical collars are sometimes informally called a dog collar.

For a more complete article on clerical collars, see: http://en.wikipedia.org/wiki/Clerical_collar

With love in Christ.

Answer 2

In the Catholic church, the white collar worn by bishops, priests, and some deacons is called a Roman collar.