Over time, most of these "dual users" tend to go back to only smoking cigarettes, according to the study published in the journal ERJ Open Research.
The international team of researchers reviewed existing data on smoking and vaping before combining the results.
After four to eight months, only three percent of dual users quit smoking, they found.
Over that period, eight percent of people who exclusively vaped quit, while six percent of those who only smoked cigarettes managed to kick the habit.
After 16 to 24 months, 13 percent of dual users had quit, compared to 26 percent of vapers to 17 percent of smokers.
Two years on, 58 percent of dual users had gone back to just smoking cigarettes, the study found.
"Given the extensive marketing of vapes as healthier alternatives to conventional smoking, they have gained popularity among people trying to quit smoking," said lead study author Josef Hamoud of Germany's University Medical Center Goettingen.
The existing research showed "concerning results" about smoking and vaping at the same time, he said in a statement.
"There is still a lot we don't know about the long-term health effects of vaping."
E-cigarettes, which have exploded in popularity over the last decade, do not contain tobacco but instead a liquid normally filled with nicotine that is inhaled as vapour.
Because there is no tar, carbon monoxide or other toxic elements linked to smoking, the health risks of vaping are considered to be much lower.
But the World Health Organization and anti-smoking groups have refused to assert that vaping is less harmful than smoking, citing the precautionary principle and saying more evidence is needed.
A 2022 Cochrane review -- considered the gold standard for analysing available knowledge -- found strong evidence that e-cigarettes are more effective for quitting smoking than nicotine patches or gum.
Fearing young people taking up vaping en masse, some countries have moved to ban e-cigarettes, or at least prohibiting disposable versions or kid-friendly flavours.
Given the mass deaths caused by tobacco, many medical researchers have called for vaping to remain legal as a tool for quitting smoking -- while doing everything possible to stop young people from taking up either habit.
