Interesting one. You can no longer create a VSO account that starts with a number. You get an error now. This was possible at one time however, and we still have accounts out there like this. Recently we heard from one who was getting an error when trying to link Azure to his VSO:
"We're sorry, but something went wrong while linking your account. Please try again"
He renamed his account to not begin with a number and the link worked just fine. FYI in case you've been with us for a long time and your account name also begins with a number.