Make Vim display a line at the edge of the set textwidth

for (g)vim, use the following:

set colorcolumn=80

or whatever width you wish. Works in both vim & gvim. I have mine within an IF so it's conditional based on which type of files I edit.

You may also use a +x/-x to base position of the column +/- from &textwidth.

set textwidth=80
set colorcolumn=-2

would effective draw the colored bar at char position 78. Of course, you may or may not set textwidth yourself, so it might be 0 (default). I use the absolute position form.

You may also change the color used if you wish:

highlight ColorColumn ctermbg=green guibg=orange

(I don't recommend THOSE colors though)

This option was added in (g)vim 7.3.

There's a snippet at Google Code you can try:

augroup vimrc_autocmds
au!
    autocmd BufRead * highlight OverLength ctermbg=red ctermfg=white guibg=#592929 
    autocmd BufRead * match OverLength /\%81v.*/
augroup END

Per a StackOverflow answer:

highlight OverLength ctermbg=red ctermfg=white guibg=#592929
match OverLength /\%81v.*/

Adjust to suit your taste.