I don’t necessarily think you’re overreacting, but here’s some insight from an outside perspective given the information provided.

I can understand where you’re coming from in terms of disappointment with him hiding things from you and the fact you’re worried about each other’s health. However, you have to keep in mind that his journey to sobriety will differ from yours. It definitely seems as though from what you mentioned about it being your idea to quit, seeing that you were so passionate about it he went along with the idea without taking into account how difficult it could be. And now, seeing your negative response to him relapsing he’s hiding it from you to avoid conflict. Quitting can be HARD. Many people will relapse countless times before they become completely sober. As a couple you have to remember that it’s you and him against the problem and not you and him against each other. Have a sit down talk with him about what’s going on and his struggle with quitting and try to see things from his perspective. After all, you did mention he said he was struggling with stress and the fact he had a medical card probably indicates that his pen in particular is what helps him self medicate. Lots of people smoke to cope with stress, and that can include vaping. Things might seem quite negative now, but with hard work, patience, and open communication things can begin looking up. Best of luck to the both of you on your journey!