It seems that there are two problems with imports from scss ressources:

• The content of files in ressources.scss is just prepended to the components scss, loosing context for relative imports (https://github.com/vuejs/vue-jest/blob/v3/lib/compilers/scss-compiler.js#L34). A solution could be to prepend @import '/absolute/path/to/file' instead.
• The importer does not support using @import 'variables' to load @import 'variables/_index.scss which should work

While debugging we also where troubled by the silent dropping of files that could not be found (https://github.com/vuejs/vue-jest/blob/v3/lib/compilers/scss-compiler.js#L27).