Hey KC what the old owner did was installed a “Blind Vent” HIGHLY ILLEGAL. The reason it is called a blind vent is when the plumbing inspector sees a pipe he doesn’t “see” it terminates inside the wall.
The proper job is installing a vent through the roof. Now some codes do allow the use of a Pro vent (NOTHING PROFESSIONAL ABOUT IT) a provent is called in technical terms air admittance valve.
Now even these lousy codes that do allow this device have certain stipulations as to location and usage (SEE your local code book) Most of the time these things have to be accessible and other great restrictions.
I would only go for the vent through the roof and if your in an area that the temperature can drop to below freezing you should increase this vent to 4″ to prevent hoar frost.
Good luck