As you suspected, you are making this too complicated. The destination doesn't need to be dynamic as the globbed path is used for the dest as well. Simply pipe to the same base directory you're globbing the src from, in this case "sass":

gulp.src("sass/**/*.scss")
  .pipe(sass())
  .pipe(gulp.dest("sass"));

If your files do not have a common base and you need to pass an array of paths, this is no longer sufficient. In this case, you'd want to specify the base option.

var paths = [
  "sass/**/*.scss", 
  "vendor/sass/**/*.scss"
];
gulp.src(paths, {base: "./"})
  .pipe(sass())
  .pipe(gulp.dest("./"));