I'd like to modify my .vimrc to read the value of a variable from an external file. How can I do this?

Specifically, a friend and I share a git repo with our .vim files, but there are a few small differences in what we want in our configs. So most of the file is common, but we use if statements to determine whether to load user-specific sections, like this:

let whoami = "user2"
if whoami == "user1"
...

After checking our common .vimrc out of source control, we each have to change the let whoami assignment so our own section will be loaded. Instead, I'd like to keep a separate file, which can be different for each of us, and from which vim will load that variable value.

Maybe another angle on this is: Will vim automatically read all the files in my .vim directory? If so, we could each put a symlink in there called username.vim, and link that to an external file that would be different for each of us.