Please do not take this lightly; about ten years ago I had a close friend who was killed by a falling stack. Read this post :
Posted by lazypup ([email protected])

:( Seeking advice for removing cast iron ventpipe - Plumbing Forum - GardenWeb

Any attempt at cutting and removing a section of this pipe from the bottom may cause the remaining portion of that stack to come crashing down instantly and without warning.

Geez I always say if it weren't for bad luck I'd have no luck at all
I guess I am luckier than suspected cause I never had that problem

Cutting in to a vertical stack without first securing it is not only stupid but idicates a death wish for the idiot that does it. When I have to make a cut into a vertical stack I place a raiser clamp, (see image) just above my cut and place a bumper jack on either side of the clamp. I then take a set of ratchet chain cutters, (snap cutters only work on horizontal cast iron) and make my cut. With the bumper jacks you can raise the stack a little if you need a little more space to insert a fitting and then lower it back down so the No-Hub Clamps,(see image) can be tightened. This is the safest way I've found to cut a piece out of a vertical stack. Regards, Tom