Alright, it's pretty clear this thread warrants development clarification, and I apologize for not getting to it sooner. Making this bit gold so it's gospel.
The Unioners and the Rheinland military do not possess a truce because they share the same faction members, or because the Union is more interested in piracy. They're presently ignoring each other as a result of the actions of the Hessians and Coalition during The Red Spring event. This event saw the diplomacy of the Unioners shift hard hostile to both the Hessians and Coalition, who betrayed them during the event. During the event, the Hessians and Coalition were able to not only conduct a prison break at one of the most secure House prisons in the sector, but were also able to cross the Hamburg system, largely unopposed, and destroy the Unioners' oldest base of Wedelburg.
With the realization that the Hessians and Coalition were able to project this amount of force within Rheinland's core systems, a truce was struck which allowed the Unioners to repressurize and fortify Vierlande as a bulwark against future revolutionary incursions through the Cologne system. Now, as to whether or not this status quo is still relevant? That depends largely upon the status of Rheinland itself. I'll present three possible scenarios below and what their outcomes would be. I want to note that at this time we haven't ended the war because if Rheinland does not go Imperial, they will need something to do, and there aren't plans to make that anything other than Rheinland continuing to gun for Saigon. This makes scenario 3 extremely unlikely.
1: Rheinland goes Imperial, the Kusari/Rheinland War ends, and Rheinland enters a period of civil war. In this scenario, it is likely that the truce would remain in place, given Unioner backing of a new Imperial government, and likely a Hessian/Coalition surge within Stuttgart and Cologne.
2: Rheinland stays Federal, the Kusari/Rheinland War continues. This effectively retains the status quo, with not enough Military forces on the home front to effectively defend their borders. It's likely the truce would remain in place (although it's worth noting that the RFP and BDM do not honor it)
3: Rheinland stays Federal, the Kusari/Rheinland war ends. In this case, it seems very likely to me that the truce would break down, and that the Military would be coming for Vierlande.