I'm really not sure about your last question - I'm just not sure. And I'm wordy, so I answered that one first...
I will do an experiment tomorrow and take it off the usb when I wake up - generally I keep it plugged in either at the computer or in the car, but I've been curious about the battery life too so I'll report back in tomorrow with how long the charge lasts.
My concern was that vaping while charging will reduce the lifetime of the battery. That's not the time between charges but how long (weeks, months) it will last before failure. Lithium Ion batteries are only good for a certain number of charging cycles, laptops etc have circuitry which prevents constant charging when they're used plugged in. I don't want to be constantly slightly discharging and recharging this if I'm vaping while plugged in and ultimately kill the battery.
To be honest, as I have a passthrough now I probably won't use it this way anyway, it's mainly the convenience of charging that really appeals to me, but it would be nice to know.