From the Pentadactyl home page:

"Pentadactyl was once called Vimperator, initially written by Martin Stubenschrott, then developed and maintained by Doug Kearns, Kris Maglione, and several other invaluable contributors. Doug and Kris, the primary Vimperator developers for several years, have left the Vimperator project and now develop and maintain Pentadactyl in its stead."

So it's essentially the successor to Vimperator, or at least a well-maintained fork by the original authors.