This is because of the fact that it is a vapor- not a smoke. A portion of the vapor itself passes into the alveoli sacs in the lungs and effectively absorbs directly into the blood stream, along with Oxygen, etc. Also, because the vapor cools on being inhaled, it condenses into a liquid form in the lungs. (thus too much vaping can actually drown you- though it would take an obscene amount.) Smoke, being a particulate only allows certain chemicals to pass through because the alveoli employ selective membranes meaning not everything can pass through. Since smoke is a mixture of particulate matter, gasses, and disolved chemicals, it becomes that only a small percentage actually absorbs.