It's hard for me to imagine people who know React moving to Vue. I'd have to assume different people, who've never known the beauty of React, are driving Vue's growth due to marketing.
I really like react, but I see the appeal of the .vue files in the vue framework. I think there are perhaps more people than you think who don't like how CSS integrates with react (CSS-in-JS) and prefer having the single file component, with super easy style scoping and with easy traditional css syntax highlighting support in a .vue file
Look at angular, the full backing of google yet people moved to react... and now to vue which was basically made by one guy