I have not heard of this before ever, I do have a theory though. Combustion is very high heat thousands of degrees F and at this temp range many chemicals get broken down before entering your body. Vaporizing is only a couple hundred degrees and most likely preserves the chemical compounds better. Its not that vaping makes it more addictive, it makes the extraction of the chemicals that interact with your brain more efficent. You are effectively getting more juice per squeeze thus increasing extraction potency.