On a toilet installation on slab floor. Where does the vent pipe go?

This is really a plumbing question not an interior question.

You may be OK already. If I recall correctly, a toilet installed on a 4" pipe can be up to 16' off the main stack without separate venting (12' off a 3" pipe). Will the plumbers in the crowd please confirm this.

I have asked for this to be moved to plumbing. I do not handle rough in work but Tom and several others do. Sounds like the toilet install is not complete. Tell us where you are at and are you doing your own work. The toilet always sits againist a wall and the vent stack is most frequently inside that wall climbing up to the attic and then the roof line. It can also be connected to another vent in the attic, called a revent.