Is there a way to reference (or "source") another user's .vimrc file?
When I kuu (a variant of su that uses kerberos security tokens) to an admin user ID, I would like to use my personal .vimrc file.
I don't want to overwrite the admin's existing .vimrc file because the admin ID is shared by multiple users.
Some distributions might add an extension to configuration (e.g. in $VIM/vimfiles) that source the file pointed by $MYVIMRC, if this environmental variable was set beforehand (normally, it's set by Vim internally after reading vimrc). This way, you won't have to pass -u each time you fire up vim. (You can of course do an alias instead, but that won't help with e.g., vipw)
Keep in mind that .vimrc can execute arbitrary commands, if you use /home/user/.vimrc you may be creating a security issue (e.g., someone manages to compromise your user account, changes your .vimrc, and then gets root the next time you edit a file as root). You can, of course, keep a known-safe copy in ~root/ somewhere.
You could presumably even set something up in ~root/.bashrc to automatically set MYVIMRC to something different for each different administrator.
But again, keep in mind that is not regular Vim behaviour nor a very common practice. Better check documentation (:h initialization and :h VIMINIT) to learn about more "sanctioned" ways of changing the location of vimrc file.