Plumbing in MA without a license can get you in trouble! For a vent to work, it must not have a belly, or low spot just like a drain. Once it is assigned to be a vent, it must slope up all the way to the atmosphere. This is to allow any condensation, rain, snow, etc., to not fill up the low spots because it can drain out. If that line is truly a vent, then you cannot run the WM drain into it, but it can be the vent if you don't have a belly in the run to it, and the distance from the trap is within 5' if it is 2" pipe. Once a vent, always a vent. Once a drain, always a drain. To change the function means you must replace it with a new run so the vent or drain function is not compromised.