I have the walls out and a hole in vertical air vent. Use a rubber sleeve to cover the hole? Don't want to replace the cast iron if not necessary. Doesn't affect venting just don't want mold, any suyggestions?

Ask for a saddle clamp in the size of your stack. Good luck, Tom

If you use the rubber sleeve coupling than you will have to cut the pipe to get it over the pipe. Do as Tom says...