Mmmm... vanilla... I wish I had discovered it before bbPress or this forum would be powered by it!
I haven't heard of anyone doing any integration, but the name/email/site come from a cookie (using PHP to load them), and I'm sure you can easily load other values if they exist. The code is in functions-controller.php in the function zp_handle_comment. Just load your Vanilla user info somewhere in there (if Zenphoto is being included inside vanilla somehow of course) and set it to the global $stored array.
Come to think of it, since that $stored array is global you can just mess with it anytime after zp_handle_comment() is called, like in controller.php. That might be more organized.
Good luck! Let us know how it goes and/or if you need more help