I would reconnect it. You shouldn't mess with FPR b/c over time the ECU will overadapt and run even leaner than before. My E30 racecar had this done and over time the car ran like crap. reinstalled stock FRP and reconnected vac line and now its MUCH better after. MB did that for a reason and the ECU is tuned assuming the vacuum like is attached. yes it will raise .5 bar w/ the vacuum not attached but don't do it, not a good idea