I developed a Gradle plugin which compiles SASS files to CSS. It uses jar files published by Vaadin. The code can be found on github. In the readme file is explained how to use the plugin.
The plugin defines dynamically a configuration and sets Vaadin libraries (SASS compiler and Vaadin themes) as dependencies. During the compilation these jars are available on the classpath allowing an extension of the default Vaadin themes.
I think it is very easy to use it, only the input (cscc file) and output (css file) are mandatory (see the examples). The plugin also provides mechanisms for additional configuration like the Vaadin versions which should be used (default versions are latest milestone from the Maven Central repository) and configuration which should be used for the compilation.
The plugin provides a Gradle task which can be used to write a custom compilation tasks with type CompileSass.