How do I Secure the Vent Stack to Replace a Section of the Waste Vent Pipe?
You should be able to hack together something to secure it, and you could probably do it from directly above your work area there in the basement, rather than having to climb up into the attic.

An initial thought would be to utilize a "band coupling" the size of the vent pipe, pushed tight against the subfloor.



Fasion some flat steel strap or angle-iron into brackets that get clamped tightly to the pipe and screw to the subfloor from below. Use several (see lame picture that shows just one strap).