While I'm sure that avoiding any unnecessary benzene exposure is the best plan, it does sound like e-cigarettes are at least quite a bit less risky than regular cigarettes. So although e-cigarettes are not the risk-free panacea that a lot of users would like them to be, they still might be the better of the two options.
From https://en.wikipedia.org/wiki/Benzene#Benzene_exposure_limit...
> The U.S. Occupational Safety and Health Administration (OSHA) has set a permissible exposure limit of 1 part of benzene per million parts of air (1 ppm) in the workplace during an 8-hour workday, 40-hour workweek. The short term exposure limit for airborne benzene is 5 ppm for 15 minutes. [...]
> The U.S. National Institute for Occupational Safety and Health (NIOSH) revised the Immediately Dangerous to Life and Health (IDLH) concentration for benzene to 500 ppm.[...]
If someone is going to smoke all day, I think that it's more appropriate to use a threshold similar to the one for workers (1ppm) or perhaps a smaller one like .1ppm.
> This estimate assumes no threshold for benzene's carcinogenic effects.
This is probably not true, but it's better to err in the side of caution, without being alarmist.