The issue still is does the cast iron pipe vent through the roof or not? Since some of the plumbing codes are different as to how far and what size of pipe you can run from the drain to the vent then I can't say anything further except talk to a local plumber and get is advice even if you have to pay him for it. 5' is usually the max depending upon the size.
There should still be a vent for the sink and tub though. Look for tees behind the wall with the vent going up. The tub and sink vents could tie together in the attic and come out somewhere else though.