Some 1.5 million vapers are ex-smokers, compared with 1.3 million who still use tobacco, a survey of 12,000 adults for Action on Smoking and Health found.
But Ash said the message that vaping was much less harmful than smoking had not yet got through to all smokers.
Some nine million still smoke in the UK despite a big rise in e-cigarette use.
In 2012, there were 700,000 vapers in the UK; now there are 2.9 million.
The main reason ex-smokers give for vaping is to help them stop smoking.
Current smokers say they do it principally to reduce the amount they smoke.
Scientists say current evidence suggests that the risks of exposure to toxins for e-cigarette users are likely to be low - and much lower than with tobacco.
Deborah Arnott, the campaigning health charity's chief executive, said the figures on vapers who had quit smoking were "excellent news" but that the rate of people switching to electronic versions had peaked.
"The rapid growth in e-cigarette use has come to an end," she said.
This is because more than a third of smokers have still never tried e-cigarettes, as a result of concerns about the safety and addictiveness of e-cigarettes.
But research suggests that 26% of people think e-cigarettes are more - or equally as - harmful as smoking tobacco while only 13% believe they are a lot less harmful.
"It's very important smokers realise that vaping is much, much less harmful than smoking," she added.
Ann McNeill, professor of tobacco addiction at King's College London, said: "The message for the 1.3 million vapers who still smoke is that they need to go further and switch completely."
People who combine electronic and standard cigarette smoking are still being exposed to the cancer-causing substances in tobacco smoke, increasing their risk of lung cancers, bronchitis and other diseases, although Public Health England believes levels of nicotine in e-cigarettes are unlikely to pose any significant health risk.
But critics say there is no convincing evidence that e-cigarettes help people quit smoking and argue they could even encourage non-smokers to start.
Get news from the BBC in your inbox, each weekday morning

Opposition factions alleged fraud during the first round of voting and there have been widespread street protests.
A special commission set up by President Michel Martelly last week is to investigate the electoral process.
There were more violent protests on Saturday when the results for the legislative vote was announced.
Jovenel Moise, a banana exporter, won the most votes in the presidential elections but with 33% fell short of a majority.
He is to face ex-state construction company head Jude Celestin, who came second with 25%.
Mr Moise has little political experience but has the backing of current President Michel Martelly. Mr Celestin is seeking the presidency for a second time.
Whoever wins will face a daunting task when taking over from Mr Martelly in February. Haiti is the poorest nation in the Americas.
Since it was devastated by an earthquake in 2010, the country has relied largely on international donations and foreign aid from the United States and other countries.

About 1,700 people were asked for their opinions on proposals which were unveiled in June.
They involved new housing, a business hub and expansion of Liverpool FC's stadium, creating up to 700 jobs.
Mayor Joe Anderson previously said it was "shameful" there had been years of delays to improving the area.
The plans, drawn up by a consortium made up of Liverpool City Council, housing providers and Liverpool FC, would also see the creation of a wide avenue through Stanley Park ending in a new public square.
About 250 new homes would be built, with 296 removed to make way, and a 100-bed hotel constructed.
Feedback from residents during a six-week community engagement project will now be considered as more detailed plans are drawn up.
They will be subject to consultation before planning applications are submitted next year.
Mr Anderson said: "People have given us invaluable information about the entire range of regeneration ideas and concepts which we unveiled.
"In the coming weeks we will use this information to refine our proposals and to undertake another listening and consultation exercise.
"We believe the plans are exciting and will deliver a massive improvement in quality of life for many thousands of people and be of major benefit to the city, not just Anfield."
For the first time, more than half of the UK's electronic-cigarette users have since given up smoking tobacco, a study suggests.
[NEXT_CONCEPT]
Haiti's presidential and legislative run-off elections have been postponed, officials have said.
[NEXT_CONCEPT]
More than 80% of people living and working in the Anfield area of Liverpool are backing plans for a £260m regeneration, according to a survey.