The study, published in the medical journal JAMA Internal Medicine, kept a track of more than 5,000 daily smokers for an average of two years.
The findings state that smokers who vaped used fewer cigarettes per day and were more than one and a half times as likely to quit completely.
But researchers also kept a track of more than 2,000 former smokers and found that those who used e-cigarettes were more likely to relapse back to smoking.
"Thus, while [e-cigarette] use can help persons reduce their smoking levels in the short term, there is no evidence that it is an efficacious smoking cessation aid in the long term," the researchers said.
However, the study also found that the risk of relapse wasn't visible in those individuals who were recent quitters.
The study took into account individuals who quit smoking as recently as 2010 and in that sample, found that vaping increased the risk of their relapse.
On the other hand, when researchers considered those who quit smoking in and after 2013, they were less prone to relapse.
According to the researchers, "technical improvements in [e-cigarettes] over time" might explain why they were less likely to relapse.
"Prior to technological advancements made around 2013, e-cigarette devices were difficult to use and only effective for the most dedicated of would-be quitters," said Gregory Conley, president, American Vaping Association.
Earlier this year, researchers in a separate study found that e-cigarettes were almost twice as effective at helping smokers quit cigarettes than traditional nicotine-replacement therapy, such as nicotine patches.
"While e-cigarettes may be a potential off ramp for adult smokers, they have proved to be a heavily traveled on ramp to nicotine addiction among youth," said Dr. Robert Jackler, founder of the group Stanford Research into the Impact of Tobacco Advertising.