The difficulty is that because vaping is, indisputably, safer than smoking, there are incentives to promote it.
It is also very, very profitable for all the vape shops that have sprung up.
It can be an effective way to wean off nicotine- it is NOT meant to be used as a “substitute” for smoking, by which I mean it should be used in decreasing doses until no vaping at all, rather than just swapping over to vaping (and increasing doses as your DH is now doing).
Aside from the tar and carcinogens in cigarette smoke, nicotine itself has serious health implications- the most prominent being that it significantly increases risk of coronary heart disease and other cardiovascular diseases. It is also a carcinogen in itself.
A major issue with vaping is that it is relatively new and so there isn’t nearly as much research about long term implications compared to smoking. However there is increasing evidence to show it causes lung damage- the extent to how long lasting this damage is is still unclear, but there are some nice visuals online that show the effects of heavy vaping for just one day and comparison to severe lung infection patients- the lungs look identical.
In reality the true danger is through first hand vaping, but breathing in second hand steam is never going to be a good thing. A major concern for me would be that children are picking up vaping at alarming rates, and smoking and vaping in children and teens is astronomically more common when the parents do so themselves as it is normalised.
In summary, a) your DH should be using vaping to cut down, not increase, and b) there is no chance in hell I’d allow my child repeated close exposure to that, due to the possible health issues but more due to the likelihood of them starting themselves.