What's the difference between pastor, parochial administrator and parochial vicar in the Catholic Church? I was looking through the list of appointments to a nearby parish and noticed that the Bishop is mixing things up, but not really changing much.  Two parishes in a nearby city are linked, with two priests taking care of souls at both of them.  They currently swap masses:

Here's an obfuscated version of the appointment:


Rev. Fr. Zack Nelson, to parochial administrator, Saint Patrick Parish, Zanesville, per canon 539, from
parochial vicar, Saint William Parish and Saint Patrick Parish, Zanesville.
Rev. Fr. Mario VanVleck, from parochial administrator, Saint Patrick Parish, Zanesville, per canon 539,
remaining as pastor, Saint William Parish, Zanesville.


So from what I can surmise they moved both priests from the arrangement where they worked both parishes and gave Fr. Zack a new title.   But, it looks like St. Patrick Parish doesn't have a pastor in this arrangement (it is the smaller of the two parishes).  

So what's the difference between the changes in status from parocial vicar to parochial administrator?  And what does it mean that St. Patrick does not have an official pastor?
A pastor is a priest with primary responsibility of the cure of souls over a parish (Canon 519)

A parochial vicar (Canon 545) assists the pastor in carrying out those duties, most often due to the size of the parish, but Canon Law also allows appointing a parochial vicar over a portion of the parish (for example a group within the parish which speaks a different language). A vicar is one who exercises authority on behalf of someone else (in this case, the pastor, which is known in Latin as the parochus).

Canon 539 requires the bishop to appoint a parochial administrator when the office of pastor is vacant and outlines the rights/obligations of the administrator (Which are somewhat more restricted than the pastor). 

Canon Law on Pastors, Parishes and Parochial Vicars

Looking at the example given, before we have two priests jointly administrating two parishes with Fr. Mario VanVleck as Pastor and Fr. Zack Nelson assisting as parochial vicar. Afterwards, Fr. Mario VanVleck remains as pastor over the one parish but Fr. Zack Nelson becomes administrator over the other. Effectively, this means there are now two parishes separately administered each by one priest. At least for the time being, Fr. Zack Nelson's responsibilities are somewhat restricted and may be intended to be temporary.

It's quite possible that Fr. Zack Nelson was made administrator as a stepping stone to being pastor. He is quite likely a younger/newer priest and so the bishop is ensuring he has sufficient experience and is capable before appointing him as pastor.