I wish most distros didn't default to nano. I like vim, I liked when that was the default, now it's such a pain running something like visudo and getting nano.
Defaults in most distros need to be sane and just work. You can change this by just copying your $shellrc with EDITOR=vim.
It's easy to change the default. It's a lot harder to teach every user how to exit vim.
Yeah if you've gone through the trouble to learn vim you're probably okay with tweaking some shell defaults as well. If you're not comfortable in the shell at all it's much better to get tossed into nano than it is vim.
I'm framing this comment.
Try using the `select-editor' command. If you don't have that, you can also try setting $EDITOR and $VISUAL in your bashrc.