I've been trying to figure this out for awhile. E-cigs are designed to produce a vapor instead of smoke, the usual application is for flavored e-liquid as its called. However I recently purchased a 'Colesman' from ColoradoE-smokes, on parker road. It's the first electronic cigarette I've used that works with a cannabis extract. It's amazing when used with hash oils. Its almost completely odorless, when you first exhale their is the tiniest hint of odor, but immediatley dissapates. Its been a great tool for me to medicate in public without worrying about smell or taboo looks.
I was medically discharged from the army, and need something for pain management and this is my new favorite device. It tastes more pure than any form of ingestion I've tried. You get the pleasent taste of the medicine but not the harsh smoke. It takes so little of the hash oil, or the dehydrated oils work very well too, but overall very cost effective, super discreet. Best money I've spent since I got my license.