Both LESS and SASS (VS2013 Update 2) is fully supported and is the industry standard for CSS preprocessors. Adding Razor to the mix would not be productive considering that both LESS and SASS are both mature, feature rich and are based upon the CSS syntax
Have you tried http://www.mindscapehq.com/products/web-workbench?
LESS is updated and SASS (.scss) editor has been added. Coming in VS2013 Update 2
Will be included in Web Tools 2012.2 RTM