Sony Vegas Movie Studio Platinum Suite 12.0 - 21:9 / 2560x1080
YouTube does not natively support 21:9 aspect ratios.  If you upload a 2560x1080 video, it will scale the video down to 1920x1080 and letterbox it (put black bars top and bottom).  You are better off rendering 1920x1080 in your video editor and doing your own letterboxing.