Sass is quite new and the tool support for it (in general, not just Vaadin) is much more limited than that for plain CSS, which is used everywhere. This is the same problem that e.g. new programming languages face - the language is often much more powerful (like Scala vs. Java), but proper tooling support lags years behind and will usually be much more limited even then due to the smaller number of active user years for the language etc.
I very much doubt you would find any nice tools e.g. to trace generated CSS back to SCSS files. That is the price you pay for the additional features SCSS brings. Structuring your SCSS well often helps a lot, though.
As for recompiling SCSS to CSS on the fly, the Vaadin servlet does it automatically when not in production mode. Just create a styles.scss instead of styles.css, then trying to load styles.css automatically compiles it. This is a full recompile, though.