If the design pressure exceeds 15 psi it is a pressure vessel within the scope of ASME VIII. I think you will find that that is consistent with the PV laws of most states and provinces.
You MUST check with the Jurisdiction on specific requirements for pressure vessel exemptions. All Jurisdictions in the US have a Department or Agency that regulates boilers and in some cases, pressure vessels. I do know that some Jurisdictions in the US do NOT regulate unfired pressure vessels.
If by "vented" you mean it operates at atmospheric pressure, it shouldn't fall under B&PV code. If desired, it could be designed under a number of various standards, including API-650, AWWA D100.
