iron vent pipe broken at fitting
If I see what I see in the bottom pic,,,you have 3 elbows running around in a giant u shape. Is this the same pipe that is rusted off in the top pics?? IF this drains water I would cut most of this out,put ONE plastic elbow and maybe a cleanout tee if it helps you any and straight from where it comes thru joist to where this pipe runs too,,,in other words cut out ALL the steel pipe and hook this new plastic,right to that cast stack threads. Even if it is JUST air vent,but the newer plastic says it drains water or it would still be steel also!! Previous poster had right idea on removing threads,,careful,dont damage it!!