ClassTree is a project mainly written in Vim Script, it's free.
Display class/interface hierarchy
This is a mirror of http://www.vim.org/scripts/script.php?script_id=1518
Define command "CTree" to display hierarchy of classes/interfaces defined
You must build tag file using Exuberant Ctags with option "--fields=+i"
Works for C#. Should work for Java and any other single-inheritance OO language.