Sort of. Over the long term, they can do retrospective research (not very good data but still data) and prospective studies. Retrospectively they can survey or see the data of every person who smokes, vapes, and does both. They can do the same prospectively. Then they can see what cancers are more prevalent in group 1 2 or 3. For example, smokers got cancer A 5x more, vapers got cancer B 3x more, and both smokers got cancer C 2x more you can conclude that cig cause more cancer A, vapes cause more cancer B, and so forth. It’s just time and data. Don’t forget causation isn’t always a direct correlation. The best example is ashtrays. People with ashtrays in their homes have much higher lung cancer rates - is it the ashtrays causing cancer? No it’s the smoking. So sometimes you have to step back and ask if there is truly a direct causation between what you are studying and the outcome