Join cast vent back into hub after removal what product to seal back together

Hi MW...

You'll want to pack in some OAKUM and seal it with a substitute lead product such as SOIL SEAL... see images below. These products are reaily available at all plumbing supply stores and even some home improvement stores.

You could also use a compression DONUT and some plumber's grease/soap... see last image. Be sure to specify standard weight (SV on pipe) or extra heavy (XH on pipe) cast iron when ordering the donut... ;)

Mark