This is the workflow I really love with Less that's missing in Sass. Killer way to use a library like Bootstrap without coupling any of your markup to Bootstrap itself.
Import a bunch of vendor styles by reference only, mix in the classes you want to keep into your own aliases, no vendor class names end up in your compiled CSS.
Sass doesn't support reference import or the ability to mixin a class, only a mixin.
// Vendor less
.btn {
// a bunch