You guys are right, it's totally legal so we shouldn't have to hide it, but if I have to get stopped by every cop in my town just to prove it's not, I'll never be on time for work again, get fired, and I won't be able to afford to vape! (much less eat, live, etc.)
The first time you are stopped, be nice. Explain to the cop what it is. Then file a formal complaint with his agency, and include in your complaint an info flyer from CASAA or something educational about e-cigs.
If you get stopped again, file another official complaint and let them know that if this continues, it will be a matter for the courts to decide, and you will be pursuing legal action.
If you get pulled over a third time, have a lawyer draft a letter saying that if this harassment continues, you will sue the Chief (or Sheriff, or whichever head LEO you are dealing with) and the officer under Federal Civil Rights Violation statutes (which means those LEOs are PERSONALLY responsible for any settlements the courts award you, and they lose their "qualified immunity", and the department can't pay the settlements out of insurance...)
The first option will get them off your back with most reasonable departments.
The second will get most mildly corrupt departments to stand down.
The third will get all but the most lawless departments to leave you alone.
If you live in Chicago, LA, NYC, Newark NJ, or Baltimore MD, you will probably have to take them to court, if they don't just kill you and leave your body in a ditch after the third complaint is filed....