Is it a demotion if a priest who was Vicar of a local County is moved to another county within the same state but named Monsignor of a local Parish? I’m referring to priests in Pennsylvania USA.
Almost certainly No
The question slightly conflates different terms, so we'll examine them each in turn.
Vicar of a local county
Presumably this is a
vicar forane, which is often known as a dean in English. The Vicar forane has a nominal position of honor over the clergy in the area (deanery) and nominal supervisory authority (he may report issues to the bishop).
Because this position is largely honorary, it's often given to one of the more senior parish priests in the area, who often is pastor of one of the more important churches in the area.
Monsignor of a local parish
Properly speaking, no one is Monsignor of a parish. Monisgnor is title for a priest as a result of an honor bestowed by the Pope. Some diocesan officials like the Vicar General automatically get the title by their position. It is typically given to priests after years of service.
The pastor of a particular parish may have the title of monsignor as a result of being designated by the Pope as an honorary prelate.
Is this a demotion? No, the title monsignor is a "promotion" (in a sense); that is, it's an honorary title on top of the priestly rank. Secondarily, the priest was ordinarily a pastor as vicar forane and still would be now (so that's not a step down). Lastly, vicar forane is a minor honorary position but more importantly they serve at the Bishop's pleasure.