As the landlord(if this is the case), be careful as they might throw the rule book at you...
a sneaky lawyer may say, " the tenant has given a verbal complaint regarding the landlord not maintaining a safe living environment."
IE: Tenant told you about his condition and that your vaping has aggravated it, yet nothing was done.
Safe living conditions are the responsibility of the land lord, as long as the tenant makes aware those concerns to the landlord.
(check local by-law/contract agreement for complete details) as i am not 100% on this.
Two of my friends are landlords, and the stories i have heard are mind blowing. Just to get out of paying rent.
best advice i can say is; have a sitdown with your roomy.
tell him that you dont want to stop vaping in the comfort of your home, but still want to be considerate of his condition, and come to a mutual agreement.
Show him some of the research you have found, as proof of your concern regarding the situation. It may even ease any thoughts that the lingering vaper that may be in the house wont kill him.
Keep it positive & keep a good friendship!