One of the things that I've found really tricky about working with Vim has been mass search and replace. Luckily I've been reading the excellent Practical Vim by Drew Neil. In it Drew explains how to use :argdo to edit multiple buffers at once. Here's how I've been using it lately and it's been working really nicely.

First, I have three aliases for working with git. These each give me pipeable portions of my git status:

# Pipe untracked files
function untracked {
  git status --porcelain | grep ?\? | awk '{ print $2 }'

# Pipe modified but unstaged files
function unstaged {
  git status --porcelain | grep '^.[^\s]' | awk '{ print $2 }'

# Pipe files staged for commit
function staged {
  git status --porcelain | grep '^[^\s]' | awk '{ print $2 }'

Then to edit files I can do the following (note I'm also using ag instead of grep):

staged | ag *_test | xargs -o vim
:argdo %s/ActiveSupport::TestCase/Backupify::TestCase/g