Sure there are downsides. Electronic cigs contain nicotine, a highly addictive stimulant. While there are tons of known carcinogens in tobacco, nicotine has until recently not been considered to be one. However, a recent study showed that nicotine gum can increase the risk of mouth cancer, which means back to the lab for the scientists who thought nicotine was safe.
Also, there's no quality control whatsoever in the e-cig industry (much of it online). An FDA report found in some e-cig cartridges "levels of carcinogens and toxic contaminants that they determined to be worthy of concern." Some users have reported problems with their lungs and throats. You'd be switching from one dangerous toxic drug to another.
More to the point, have you really tried everything else? I have it on good authority that hypnosis and EFT both work for quitting smoking, as long as the person is highly motivated. Perhaps it's worth your time to look into these alternative methods. Good luck and wishing you all the best. :)