Vanilla is fine for small projects (and I tend to prefer it for static websites), but if you try to develop a large web app using vanilla JS you'll just end up

Vanilla is fine for small projects (and I tend to prefer it for static websites), but if you try to develop a large web app using vanilla JS you'll just end up making your own half-baked framework. A kind of variation on Greenspun's Tenth Rule might be in order: "Any sufficiently complicated web application contains an ad-hoc, informally specified, slow, bug-ridden version of half of React".