Most active development happens on the vim-dev maillist.
Subscribe if you want to get involved, see
To see the known bugs and next things to work on, use the Vim help:
There is also an issue tracker
Making changes to Vim
Since Vims source code is hosted at github.com and managed by git it is most
easiest to use git for developing and bugfixing Vim.
See The GitHub page.
If you prefer to use Mercurial over git, there is an alternative:
the Mercurial mirror