FYI: Excellent Stealth Vaping Technique:
disclaimer - I didn't discover this, I saw it posted somewhere here on ECF; but haven't been able to find the post to give credit to the original poster.
I've used this technique with 60/40 PG/VG and had very little vapor on exhale.
Take a pull off your ecig, as big as you want; but leave enough room to inhale air through your nose when you're done.
That's it. Not sure why this works. But if you inhale air through your nose before exhaling the vapor, there is little to no visible vapor exhaled. This doesn't work as well if you inhale air through your mouth before exhaling the vapor. And there is no need to hold the vapor for any extended period. Vape - breathe in through your nose - exhale.
ETA: Dang, finally found the post! From BigJohnJ -
http://www.e-cigarette-forum.com/forum/e-cigarettes-public/160056-invisible-vaping.html#post2654912