A two year study on vapers published in the BMJ finds that smokers that successfully switch to vaping are less likely to go back to cigarettes (61% still vaping).

Cohort study of electronic cigarette use: effectiveness and safety at 24€…months -- Manzoli et al. -- Tobacco Control

The conclusion is that if someone quits smoking using a vaporisor then there is good chance they will not go back to cigarettes. Traditional NRT can not boast a figure like this, and we all know it has a much lower success rate even at 1 year.