I did a home today where the exhaust from the vacuum system appears to run into a floor fitting that may or may not be connected to the sewer system. My gut feeling is that it should not be connected to the sewer system or any drainage system and I am trying to find something that says this is not correct.
Photos, since what you are saying sounds very ‘weird’.
No central vacuum system should ever be connected to any sewer system. My company installed hundreds of central vacuum systems over the years as an authorized dealer and I can promise you they do not get connected into the sewer system.
Some newer ones have a WetVac feature but this is not something that gets dumped directly into the sewer system. Sad to say I can see how easy it would be to modify one of these systems to do that but I dont believe it is manufactured that way.