Smokers can improve the health of their hearts within weeks of switching to e-cigarettes, the largest trial of its kind shows.
The month-long study of 114 smokers suggests vaping has the potential to reduce heart attack and stroke risk.
The team, at the University of Dundee, stressed vaping was "not safe" - just less harmful than tobacco.
The British Heart Foundation said stopping smoking was the single best thing you could do for your heart.
Chemicals in cigarette smoke narrow arteries as they get furred up with fatty deposits increasing the risk of a deadly blockage. Ultimately smoking doubles your risk of having a heart attack.
But the researchers said the current evidence on vaping was "very poor" and often assessed the impact of a single e-cigarette on heart health.
So they monitored people's blood vessels a month after they were switched to e-cigarettes on the trial.
They focused on how blood vessels expand when a wave of blood rushes through, by measuring "flow-mediated dilation".
The more the blood vessels are able to expand the healthier they are. Flow-mediate dilation scores have been closely linked to the long-term risk of heart attacks and stroke.
The results, published in the Journal of the American College of Cardiology, showed:
So, switching to vaping got those smokers about halfway back to a healthy score.
"They're not quite normal, but their vascular function improved quite significantly, just within a month," said one of the researchers, Professor Jacob George.
The study is too short to fully establish whether or not this improvement could be sustained in the long-term or if vaping would definitely save lives.
It is also worth noting that vapers did not have a normal score.
Prof George added: "The key take-home is these devices are not completely safe and should not be tried by non-smokers or children.
"We now have clear evidence they're less harmful than tobacco cigarettes."
The potential dangers of vaping were highlighted this week when a British teenager told the BBC how e-cigarettes nearly killed him.
The devices set off a catastrophic immune reaction in his lungs that left him on life-support with his breathing replaced by an artificial lung.
But overall, the advice in the UK is that vaping is 95% safer than smoking and that smokers should switch and non-smokers should not take up vaping.
The British Heart Foundation said 50 people every day die as a result of heart problems caused by smoking.
"Stopping smoking is the single best thing you can do for your heart health," the charity said.
Prof John Britton, director of the UK centre for tobacco and alcohol studies at the University of Nottingham, said: "This randomised trial provides clear evidence of a reduction in a marker of cardiovascular disease risk in people who switch from smoking to vaping.
"The finding of the study, that vaping is less harmful than smoking, is intuitively correct on the grounds of the lower range and levels of emissions known to be present in vapour relative to tobacco smoke."