Where should the main stack vent go?
You have one main stack typically.  All of the plumbing drains into it and goes down.  The venting for the plumbing should be above the highest drain-to-main-stack connection.  The venting goes up - on a typical home it would vent out the roof.  If you had the vents below then you may get water/sewage in them and your house would stink and possibly drains would not function as well.  If you are more specific I can touch on how to do this right.