Google Code is shutting down this year[1], so the Vim-dev mailing list discussed migration[2]. GitHub is the most popular host for open source projects, and Google Code has an export tool for it[3], so the switch makes a lot of sense.
That said, Vim's development method is rather conservative. Vim was created in 1988, but source control wasn't used until 2004 and Bram is the only one with write access. There are no pull requests. You have to send patches to the mailing list, where they are often dropped or forgotten. Hopefully, GitHub's features will modernize things a little and make it easier for more people to contribute to Vim.
That said, Vim's development method is rather conservative. Vim was created in 1988, but source control wasn't used until 2004 and Bram is the only one with write access. There are no pull requests. You have to send patches to the mailing list, where they are often dropped or forgotten. Hopefully, GitHub's features will modernize things a little and make it easier for more people to contribute to Vim.
1. http://google-opensource.blogspot.com/2015/03/farewell-to-go...
2. https://groups.google.com/d/msg/vim_dev/ehzfCDccmek/PU1sTZNd...
3. https://code.google.com/export-to-github/