I don't know if "from scratch" is really a fair statement. React accepts HTML in the render function, so the only thing you'd need to do is modify the variables to match JSX's variable syntax, which is exactly what you'd have to do with Vue.
I think it's all pretty much the same deal. You might argue Vue's template syntax is closer to HTML than JSX is, but that doesn't really feel like the major issue when slowly refactoring a large application.
I'm not talking syntax. I'm saying that vue can be added on top of a third party DOM that may have server-rendered data embedded in it. You cannot hydrate React on top of dynamic HTML that was generated with JSP/PHP/.NET/etc unless you a) give up JSX (a big departure from idiomatic React) and b) interpolate server variables into your JS (a no-no in pretty much every non-SPA framework)
For example, take a rails app, then do the equivalent of $('#foo').toggle() based on some condition. In React, AFAIK you can't unless React knows what the markup being toggled is somehow. Vue can do it without knowing anything about the markup inside, and more importantly, the code will look idiomatic.
> and more importantly, the code will look idiomatic.
That's the kicker.
Whether a thing is possible or not is a far cry from whether it's a good idea. And when you consider what someone who opens up the code 5-7 years later without any frame of reference to start from will think (which can just as well include the original author if the original author has moved on to other things), familiar syntax goes a long way.
I think it's all pretty much the same deal. You might argue Vue's template syntax is closer to HTML than JSX is, but that doesn't really feel like the major issue when slowly refactoring a large application.