I got a bit tired of vim being a bit rubbish when you first use it on haiku (compatibility mode, backspace not working, etc etc)
To fix it try a vimrc with the following contents:
set nocompatible syntax on colors darkblue
filetype indent on
filetype plugin on
setlocal spell spelllang=en_gb
map :set spell!
And save it to /boot/home/config/settings/vim/vimrc
This vimrc will also set the spellchecking language (but set it to spellcheck off by default and press ctrl-j to toggle it on/off), turn on syntax highlighting, auto-indent, highlight all search results, highlight whitespace at the end of lines (or empty lines with whitespace on them) and a few other things.
Hope this helps a few people out!