I am plumbing a basement bathroom. I have a 3" stack for my toilet roughed in, can I reduce that to 2" to tie into a vent that is close to the bath area or should I tie into the 4" main stack

I have a 3" stack for my toilet roughed in, Please give me your definition of ' stack". What's picking up the toilet? re you asking if you can cut in a 3 X 2 inch fitting to a 3" toilet drain line and tie backto a existing vent? Could you draw up a diagram or explain your question farther? Regards, tom

Yep, going to need some more info here. A two inch vent is fine for a toilet, but it all depends on what you are going to tie the vent into. If you reduce the three inch vent(for toilet) down to two inch and tie it into another two inch vent(depending on how many fixtures are already vented off the two inch line) you should be fine. Can you please post some pictures of your layout. It would help. And please let us know exactly what vent and drain lines sizes are for the work you would like to do.