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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.