pulling the cover plate ( its just a cover, the old stopper linkage is gone) behind the cover there is a elbow at the top of a pipe with a flange facing the tub. the flange is set at a angle and stands a half inch off from the tub. the angle the flange sits at, doesnt match the slope of the tub. the gap between tub and flange is wide enough i have no problem seeing the washing machine in the basement below. and my kids have no problem splashing a gallon or two of water down on top of the washing machine.
searching the plumbing aisle at hardware stores and DIY stores, i didnt find any magic do-da. It looks like i can cut a gasket to try to squeeze in there or replace the section from the back of the tub to the top of the trap. the gasket seems like the simplest fix, but im not sure how well i can bond it.
Is there a simple way to go about this, a magic do-da i might have missed.
What you're describing is the tub overflow and toggle drain stopper assembly, not a vent. Get a new overflow toggle bolt plate and overflow assembly. There is supposed to be a thick rubber gasket between the outside back of the tub and the overflow, and you need one of those, also.