You end up getting more THC out of a vaporizer than from smoking any other way. In the most efficient vaporizer, most often the Volcano, you can get up to 95% of THC by using it. Whereas using other methods like joints, blunts, pipes, etc, all cause the weed and THC to burn, literally, and can cause up to 50% reduction in THC intake, versus the 95% possibility when smoking through a vaporizer. Not to mention, it's incredibly safe. It eliminates all but a few chemicals, and you literally don't even see any smoke, or barely any at all. The high is incredibly more intense due to the larger intake of THC, so you can smoke some of the same weed in a vaporizer that you did in a joint, smoke less of it, and get more high just by using the vapo.
It won't set off a smoke detector. It doesn't eliminate ALL of the smoke, but it definitely gets rid of a ton of it. I would say though that yes, in most cases it practically almost eliminates the smoke.
Problem is, the Volcano runs around $300+.
Sounds like there's some major conflict of interest issues going on here. You need to get over the fact that people, including your friends, smoke pot and that it is now generally accepted as a safe practice, especially by young people. Alternatively, they need to realize that there are people out there, like yourself, that do not like Marijuana or anything to do with it. For whatever reason, both sides see each others arguments as stupid. It's a never ending debate that both sides have to learn to deal with. Unfortunately for the propaganda pushers, they are now losing to scientific fact. But still, it's too bad for you, and it's too bad for them. They're going to smoke whether you like it or not, and you're going to shun them for it whether they want you to or not. And the police probably wouldn't care enough to come over to your place just because a few teenagers are smoking pot in their own home. Depending on where you live of course most definitely (you said UK if i recall correctly, so I cannot say). Although, regardless of that fact, it sounds like everyone needs to relax and sit down and discuss things. If that can't happen, then your next course of action should pretty much be obvious.