How do you search all source code in Vim?

Set grepprg to Ack, then use :grep to search with Ack.

Or use ctags.

You may want to check out :vimgrep and :grep in the vim documentation. :vimgrep uses vim's own pattern searching functionality and reads all files into vim buffers. :grep by default links to an external grep utlity on your system, but you can change the behavior by setting the grepprg option. Here's a link to the online docs:

http://vimdoc.sourceforge.net/htmldoc/quickfix.html#grep

There's more info on that and also some other options in the tip on "Find in files within Vim" at the Vim Tips Wiki:

http://vim.wikia.com/wiki/Find_in_files_within_Vim

:vimgrep "pattern" ~/trunk/**/*.py
:copen 20

If you have quite a big project I'd recommend to you to use cscope and vim plugins. Here is one designed to handle big projects: SourceCodeObedience

There is a brief description of how to grep text using SourceCodeObedience.

I use grep directly for that.

grep -IirlZ "pattern" .|xargs -0 vim

-I: ignore binary
-i: ignore case
-r: recursive
-l: print file name only
-Z: print \0 after each file name