If the judge rules to vacate a POS, is that a good thing for me, the plaintiff?

No, it is the exact opposite. The judge ruled in favor of the Defendant that your proof of service was defective. To make proper service just mailing it to attorney if attorney has already appeared in case, but if Judge ruled on own that it was improper you must properly serve the actual defendant unless their attorney is willing to accept service. The law is very simple and clear [except as to businesses] so if you had problems with this procedure you likely will have some more major problems later. You ideally should at least have an attorney who is willing to give you general advice on how to proceed; some of us semi-retired attorney are willing to do so, but most attorneys do not consider it worth what they can charge versus the risks they incur of malpractice claims.? There are various books that can help you out.