Upgrading legacy applications was one of the usecases Vue was designed around. It means that developers can piecemeal upgrade bits of an application as necessary.
In my experience Angular, React, and a lot of other frameworks ultimately require you to go all in early and establish a large toolchain around these frameworks. Angular prescribes a lot with its amazing CLI. React on the other hand doesn’t prescribe anything, but requires you to self-assemble and wield a somewhat complex toolchain. But as Evan put it in his JSConf Asia talk, Vue sits in the middle of the “Cathedral and the Bazaar”. Vue has useful tooling, but it’s all optional and you can use only what you need. In some ways, Vue’s grafting capabilities really does make it seem like a jQuery replacement you can drop in to give your components superpowers as needed.