The Ultra Brief Guide to Vim is part of the very young Brief Guide series of documents, once found at http://brief.collapsar.net/. The Ultra Brief Guide to Vim itself was at http://brief.collapsar.net/ubg/vim.txt.
It's the precursor to VimVim?, RicardoSignes's long-promised crash course on Vim for people who've used Vim for ages without learning any of its second-order features.