They are not in holy orders; the term "vicar" is derived from the Latin adjective vicarius ("substituted") and in this context simply means a deputy.
The title refers to the laymen who were employed to sing musical sections of church services during the Middle Ages.
This collegiate establishment at Hereford was most unusual; the only other cathedral with a similar body was St. Paul's where there was a much smaller college of minor canons.
[1] With the post-war proliferation of Cathedral choral scholarships, however, many cathedral or collegiate choirs comprise a balance between choral scholars (or, as at New College, Oxford, and Magdalen College, Oxford, "academical clerks") – university or "gap year" students who combine their studies or other commitments with singing – and lay clerks.
Undergraduates admitted to a College on the basis of their ability and potential in both singing and academic study are traditionally called "Choral Scholars".