But, vite is working on to become the next nextjs, like ssr, isg etc. Nextjs only support react, so it's easier to leverage the react's ability to do these.
When there is ssr, there is runtime, more than just a dev/build tool.
It sounds like you might have some misunderstanding. You can write SSR applications with Vite, but each framework or application is responsible for the rendering. There isn't really such a thing as an SSR runtime in Vite. It will run code on the development server in SSR mode, but all it does is make sure it's picking up code updates and passes it through to Node.js.