Does the 8 foot lateral clearance to sidewalls apply to PVC Cat IV vents? I did not see anything in the codebooks to suggest that it does not. However, I don't see anything in the manufacturer's instructions regarding this.
Single bowl sink. Install an Air Admittance Valve - AAV (aka Studor vent). Water may be getting sucked out of the P-trap every time you drain the sink, or other drains are using the sink drain for venting. Is roof vent clogged?