ᐅ Reconnect old drainage stack with ventilation to prevent negative pressure

Hey, I drilled the drainage pipe from the attic floor (DG) into the second floor (2 OG) and connected it to the toilet there to create a toilet connection for the attic. (Starting from the second floor, only an 80mm (3 inch) pipe was used, which is therefore unsuitable for a toilet.) Now, regarding the pressure: Should I install a vent pipe going upwards as well, or is it enough if the old drainage pipe serves as a vent and pressure relief?