You can start with the Wikipedia article. The problematic part here is that we have no long-term studies about vaping and even less studies about people who exclusively vape. There are plenty of studies that indicate that there is a lot of nasty stuff mixed in the aerosol that vapes produce, like this one.
Another thing is that people who are selling these are the same people who sell tobacco and they have decades of experience mudding the research field.
I in general despise vaping as it made a lot of younger people pick up nicotine and it is far more addictive when you are young.
And, well, yes, I am exaggerating with that statement, but the problem is that we don't know for certain. Substitution therapy exists, but I just can't support it when whatever you are using is wildly underresearched.
Yea, THC vapes are also illegal in most jurisdictions. People still buy and use them.
Yet I know of no studies that would look into health of people who are exclusively vaping across even a 5 year span. It's not "some unknowns", we barely know anything about it.
This is one of the most wild takes I've heard. Something being safe to eat or put on your skin does not mean you can inhale it and expect no harm. One of the most common examples would be benzaldehyde, which is used as a food additive and in vapes, as a gas it causes irritation to respiratory passages.
The worst part is that sometimes you don't even have to specify a lot of it in the list of ingredients. You can read article 4.2.2.1 of this document, for example.
And nicotine patches beat both of those by a mile in this regard.
The best way of lowering the risk is using proper, well established methods for quitting smoking before resorting to something like vaping.