As far as I know, there's nothing quite like emacs org-mode for VI.

But you might want to check out this bunch of scripts for plain text GTD from the command line:

http://todotxt.com/

A great outlining tool is here:

http://www.vimoutliner.org/

You could use Vim or Vim Outliner with GTD by adding context and/or project tags to each line/task. For example

walk dog p:dogcare @home

You could also add priority codes if desired.

Then simply grep the file to pull up all the lines associated with a particular project and/or context.

VimOutliner would work well for this because you could do hierarchical, check-box project planning in the outliner and then search (grep) it by context.