Venting Plumbing AFTER drywall is up
To vent plumbing you must either find a way to tie into an existing stack vent, go to the outside via wall (this is code depending on area and must be higher than all fixtures - what you are describing is not), or go through roof.  

If the roof is available this is probably your easiest option.  You drill a hole and throw a roofing vent with gasket over the hole, caulk gasket, plug pipe in bottom of vent through hole and you are done.  This is a very easy, forgiving process.   With materials in hand you can do this in 30 mins.  So just do it right.