> having smoke in your lungs is having smoke in your lungs whether it comes form a cigarette or a joint

This is not a thing that's true, it's a thing you assume. Having different smokes or vapors in your lungs can have different effects, just as drinking different liquids can have different effects. Tobacco smoke causes cancers. The more elements in another smoke that are shared with tobacco (or other plants that cause disease) the more the new smoke should be suspect, and it suggests a direction for initial testing. We can't just treat all odors as cancer-causing; vaping doesn't even involve burning vegetable matter, so even if burning some chemicals common to all plants causes cancer, that wouldn't make vaping suspect.