I'm a criminal defense attorney. If you are confronted by the police about your PV or if the PV was the reason for the initial encounter, then your best defense is to be polite and explain to the officer what the device is. Let's not kid ourselves, some PVs look like drug smoking devices. The real question is does an officer have the right to stop you, either while driving or just on the street, if he/she sees you with your PV. An officer must have reasonable, articulable suspicion that a crime is, has, or is about to be committed before the officer can detain you. I think because most people, which includes officers and judges, are not familiar with e-cigs, then it would probably be deemed reasonable for an officer to stop you if he/she saw you vaping. Of course, this will never be tested in court until someone is stopped because of his or her PV and then arrested for something else, i.e. "real" drug possession, suspended license, etc. Then, some attorney, like myself, would file a motion to suppress the stop and all evidence seized arguing that a PV is not a reason to detain. But as I said, until the public is better educated, I think I would lose that argument because so many PVs do look like a mechanism to ingest an illegal substance.
So, education is key. Being respectful to the officer and not acting like a jerk will go a long way, too.