When I had a rental a few years ago from Enterprise it was made extremely clear that No Smoking was allowed and clearly explained that you would incur a hefty charge for cleaning the vehicle. I asked the rental agent about E-Cigs and he look at me puzzled. I explained it was no flame, no burning and no smoke or odor then I demonstrated in the office and he said he did not see any problem. Not sure if they have changed their policy since then but that was my experience.