When Im working a flight, I love seeing passengers vape. I just tell them not to flaunt it, no smoke rings or anything, and IM totally kewl with it. Even though I vape, I dont need a bunch of Nic fitting passengers while working a 14 hour flight to the Middle East!!!
As far as what happens when the smoke detector goes off... In reality not much. You will experience the most AWEFUL and Ear Piercing sound you have ever heard, and probably will have a frantic Flight Attendant ripping the door off the hinges. Some aircraft the lav detector is linked to a a sensor in the flight deck to alert the captain something is up. Most of the time, when they go off, the flight attendant simply finds the source of the alarm (A smoking passenger, or a vaper that blew too close to the detector), resets the alarm, and deals with the issue.
Heres something just for laughs....
I was working a flight we were chartered for out of poland. We were taking a whole plane full of polish nationals that spoke NO english, and none of our crew spoke polish. We had 19 passengers that lit up analogues in the lavs, they knew what they were doing cause they even tried (And some succeeded) to take the batteries out of the detectors!!! Needless to say I had a lot of paperwork to fill out after that wonderful day!
Safe Travels!!!