The e-cigarette is an inhaler and the commercial brands do not appear to deliver nicotine deeply into the lung. Its ability with other drugs is untried. Nicotine by e-cigarette is probably absorbed from the throat and upper airways rather than from the lung.

People mixed tobacco with marijuana for smoking long before e-cigarettes, and smoking is the most dangerous way to consume cannabis or tobacco.

Devices for heating and vaporizing cannabis long preceded the e-cigarette. Whether or not e-cigarettes could be used for that purpose, no smoke will be inhaled. 

It is not known whether the e-cigarette would facilitate absorption of THC, the active ingredient in cannabis. The e-cigarette vapourises liquid but does not burn plant material such as marijuana or tobacco. If cannabis is vaped by e-cigarette, rather than combusted and smoked in a joint,  then no smoke is inhaled. Lung cancer  is likely after many years of smoke inhalation, of either tobacco or marijuana smoke.

E-cigarette vaping of cannabis is likely to cause far lung damage than smoking.

For example, the illegality of cannabis makes adulteration possible, and prevents legal controls on its production. Also research safeguards are lacking. The effect of inhaling and vaporizing THC in the e-cigarette liquid has not been tested and researched by anyone to our knowledge.