Bathroom vent stack
I am in the process of remodeling my bathroom, and when I opened a wall to incorporate the closet on the other side, I found the house's main vent stack. It goes from the basement to the roof and services all the house's plumbing. It is a 5 or 6 inch cast iron pipe, with a few connection in the wall to be taken out. My question is, can I cut this pipe just below the floor and top it with a Tee then route it over a few feet left and right, then up the exterior and interior walls of the house, then back to the original exit in the ceiling. I would be splitting the pipe into two section to facilitate the venting of a shower and sink on the interior side, and a toilet and the vent pipe from the bathroom below. All the new pipes would 4" ABS.
It is NOT 5" or 6" if this is merely a residence. As for the rest of the question there is no way to give any answer because we would have to at least see a picture, and possible the actual house to tell you.
Can't tell from your description, but you can NOT drain the new 2nd floor bathroom into the pipe which is venting the first floor. If you use that pipe as the waste, then a new vent from the first floor to roof must be installed.
A couple of other considerations...cast iron is quite heavy and in some places, it's against code to insert plastic somewhere in the middle of a vertical run. If it is truely a vent, as long as the offsets have some slope, you could probably do it. Since rain or condensation could end up in the pipes, they must slope just like a drain line does, but the air doesn't care. It might make it nearly impossible to clear out if for some reason it got clogged with that offset in it. The drain pipes are sized by ID...a 4" CI pipe would likely be in the order of 4.5+" in diameter OD. Making that vent a drain, though, as noted, requires additional work for things to operate properly.