2.6 has the 1.3.3 version of the official LESS compiler. It might have a bug. I'll investigate this issue and release an update when I have a fix.
Thanks for reporting this!
UPDATE: FIXED!
This has now been verified and fixed for VS 2012 as of July 16, 2013.
Version 2.8 of Web Essentials addresses the issue in this question, so if you update again from within Visual Studio 2012 or by manually installing the extension you will be able to use the LESS compiler for relative @import statements.
It will only work with LESS 1.3.3 (the version packaged with the extension), but this shouldn't be an issue since it now works out-of-the-box.
To see what else was updated in v2.8 of the extension, check the change log: http://vswebessentials.com/changelog
Thanks to Mads Kristensen and the rest of the team that helped to fix this!
UPDATE: VS AUTO-UPDATE
v2.8.1 of the extension for VS 2013 pushed out to the Visual Studio Extension Gallery. You should get the VS auto-update message or go to "Tools > Extensions and Updates..." menu item and update from there.