Even when we don't have the numbers for everyday long-term use we already know the chemical and biological mechanisms that make smoking so harmful and carcinogen and we can measure that these substances are almost entirely absent in vapor during normal use of a vape because almost all of the problems with smoking come from burning tabacco and inhaling the fumes and vaping is simply way cooler and there is no fume but vapor (hence the name) and we know that the potential of vapor to do harm is way lower than fume. We never have any long-term data for anything new. But to make an argument for not consuming something you would at least have some reasonable mechanism through which it could be harmful and a lot of those known to be harmful mechanisms are present when smoking but absent when vaping. Even the British National Health Service agrees that vaping is at least 95% less harmful than smoking and this is a reasonable position.
> Trading the terrible effects of burning tobacco versus the insane amount of nicotine from vaping seems like an unknown tradeoff to me
This is just black rhetoric that is not evidence-based. How do you know the amount of nicotine is "insane" during normal use?
All available evidence points to vaping being at least one magnitude less harmful than smoking and this is backed by what we know about chemical and biological processes of the chemicals involved and through medical reports about people who replaced smoking with vaping. It would be irresponsible to deny smokers who can't quit a way to harm reduction just by claiming we don't have any long-term data. We know enough about combustion, vapor and the chemical and biological implications to be quite confident that vaping is much less harmful than smoking.
That doesn't mean we should promote it to anybody who is not already a smoker. But as a means to reduce harm for someone who seems unable to quit smoking it should be considered.