I was wondering if I installed an in line vent would I have to leave an axis to replace it in the future?

By "in line vent" are you meaning a AAV Air admittance valve - Marley Plumbing & Drainage (http://www.marleyplumbinganddrainage.com/diyer/content/2/226/air-admittance-valve.html) If so, then yes, they fail eventually.

I guess u can call it a AAV as well. The thing is that I plumbed a bathroom and I have no way to penetrate through the roof with my vent. Therefore I am going to install an "in line vent" which purpose is to work as a vent by installing it 6'inches above the flood rim on the highest fixture without any corrosive wastes, but I don't whether to leave an axis in case it begins to malfunction rather then leaving it buried behind drywall.

"I don't whether to leave an axis in case it begins to malfunction rather then leaving it buried behind drywall.
" reread my post #2, now you do know. They will eventually fail and its againist code to seal them inside a wall.

Agree with ballengerb1. All AAVs or studor vents need to have an access panel, or otherwise be READILY Accessible. An inline vent(aav, studor vent) has working parts (moving parts) that will fail over time, so you need access to change it out when it does.