E-cigarettes / vaping products are intended to be used by adult smokers as a way to reduce harm, not by people who do not smoke.
There is no such thing as absolute safety - but just because we don’t know everything, does not mean we don’t know anything.
Nicotine does not cause cancer, nor does it contribute to respiratory diseases. Like caffeine, nicotine is a stimulant drug and it may be that people with underlying heart conditions would be advised against regular use of any stimulant drug.
We do know enough about the relative safety of e-cigarettes / vaping products for doctors to recommend them to people who cannot give up nicotine and/or for people otherwise unable to quit cigarettes. That’s because evidence consistently shows that compared to smoking, e-cigarettes / vaping products are at least 95 per cent less harmful.