All you have is statistics. Smokers are X% more likely to get certain diseases. The second hand smoke argument evolved in the same way. The lawsuits that gave it legs involved people like airline staff and bar staff who were exposed to large concentrations of it for long periods, and they were statistically more likely to have certain medical conditions.
That allows morally shameless folks like big tobacco executives and Rush the Drugster to assert that maybe it's not a causal relationship. People with a tendency to smoke or work in bars are more likely to develop COPD due to some unknown underlying condition that causes both the behaviors and the disease. Or it's a liberal lie devised by them "scientists" or big government or whatever.
God help us vapers. There isn't a large enough database yet to get us to whatever degree of certainty we have about smoking. Which allows anyone with a theory or a financial incentive to make whatever claims they want, then turn it back on us by asserting that we don't have any "evidence" that they are wrong.
Like JDM said above "believe what you want to believe". I've got my own home brewed mix of common sense, "facts", and anecdotal evidence, which leads me to believe I'm a lot better off
vaping than smoking. I'm not going to put much effort into persuading people that I'm right. Except in the limited area of trying to keep vaping legal.
I have a feeling that if gravity had been "discovered" in recent times it might not have caught on like it did. People don't believe politicians, or scientists, or polls, or logic. But they believe anything their witch doctors tell them. I heard it on Rush, you know. Is human culture devolving?