I don't want to delude myself. Crowd sourcing.
...Because if we're all deluded as a group it feels better?
We can't tell, we really can't tell. There's no long-term info on it because no one has used them long-term yet.
I just want to take the time right now to point out that DOW chemicals (makers of like, most of the propylene glycol out there) does NOT recommend using PG in theatrical fog machines because of lung irritation, and also recommends that those using fog machines limit its use to performances when possible. A Swedish study (this is from the wiki page on PG, reference 37) found a "strong" connection between concentrations of airborne PG in households and children developing asthma and allergic reactions.
A vaporizer is a hand-held fog machine that we're sucking straight off the tap, so... chew on that before you think it's truly "safe".
I don't want anyone to think I'm trying to talk vaping down. I'm not. I'm sold, hooked, and would never go back. I whole-heartedly recommend it to smokers.
That said, I'm not about to pretend there are no ramifications to vaping. And I'm certainly not going to put a % on it. What exactly does it mean to be 50% less safe??? If a disease makes the same fraction of people sick but the symptoms are half as severe, is that 50% more safe? What about one that hurts just the same but has a 50% less chance of infecting you? See what I mean?