The reason is because vacation days have to be requested but (three) PTO days essentially can't be denied by the employer. It was part of the bargained agreement when we accepted their offer to eliminate sick days.
I don't see where the employer's procedure is breaching the contract so if you aren't happy with the procedures you renegotiate the contract at the next opportunity.

I get that employers want to have complete control of employees' schedules but if my child wakes me up at 3:00am to tell me that he can't breathe, I'm going to take him to the hospital and miss a day's work, no matter what my employer thinks (again, maybe that's just me).
I'm with you on that and I would hope that every parent feels the same.

If you live in world where employers have a concern about their employees, that is awesome! I don't live in that world.
Neither did I. I think they are hiding out somewhere with the Unicorns and leprechauns. I spent 40 years in the working world and never found any employer that had any concern for anything but profit and would use their employees as slaves if they could get away with it.

I don't have enough time to tell you all of the petty, vindictive things that I have seen supervisors do to subordinates that they dislike/don't respect. If you've ever worked in a governmental workplace, you know how incestuous the hiring practices are. And when you have family working together (especially when one is a supervisor and one is a subordinate), all of the inter-family dynamics play out in the work place as well as the home life. It can be exhausting dealing with it. If you want to see pettiness, just come to my work place.
I've worked for some pretty crappy employers but I always had the luxury of having no vested interest in the company so I could always find another job and quit. It's not that easy working for a government entity if you have pension and seniority issues but it's still an option. You have to decide whether the job is worth being miserable.

I was hoping that a similar case may have been adjudicated
You would have to do appellate case searches for your state. Google Scholar is a good place for laymen to look up case decisions. Try that if you like but I can tell you from experience that figuring out the search parameters for this topic would be time consuming and difficult.

Our union does have attorneys. However, I have no access to them except through my union rep,
I guess I don't know much about union hierarchy. You wrote that you were an officer. I would have thought that would make you superior to a rep.