I don't think a vicar should have to do any more work than any other professional. What is unusual is that their hours and not typical of other professionals and they need to be flexible.
If you think about what a vicar has to do, it is the Sunday service, which probably takes about a day's preparation - plus a midweek service - so make that 2 days' worth of work right there. If they have have a different sermon at an evening service, then you might be adding on another day's worth of work.
Add a couple of evenings taken up with committee work - there's another day's worth of hours. We are now up to four days' worth of hours.
Then add school's visits, day centre visits, work place visits, funerals and weddings.
Then all the pastoral work - visiting people in need, meeting up with new families, baptism candidates, funeral preparation, confirmation classes.
Now add any non-Sunday training/bible study/courses, as well as their personal study.
And then all the behind the scenes stuff that is not known to members of the parish.
It is clear that there is far too much work for one person. It has to be delegated.