@devinrm don’t discount your contribution! This is great advice, and using a dictionary to autocomplete is a neat idea.
You might want to consider contributing the dictionary idea (and use setlocal complete+=k...) to Keith Smiley’s rspec.vim (not to be confused with thoughtbot’s vim-rspec), which provides rspec-specific syntax highlighting. Being able to autocomplete shoulda-matchers in RSpec seems like a good idea.