Telling Vim to use custom .vimrc is easy, but how to tell it to use alternative path instead of ~/.vim?

Vim looks in ~/.vim because that directory is in the default list in 'runtimepath', abbreviated as 'rtp'. To tell Vim to look elsewhere, you will have to completely specify a different 'rtp' value, or edit the default using a call to substitute() for example. If you just want Vim to look in a different place for your configuration files first, don't care that it also looks in ~/.vim, and don't care that it doesn't look in your alternative after directory, the command is pretty simple:

vim --cmd 'set rtp^=alternate_dir'

See

:help --cmd
:help :set^=

Replacing .vim with your alternative directory takes a little more typing.

vim --cmd 'let &rtp = substitute(&rtp, "\.vim", "alternate", "g")'

I tried replacing ~/.vim with another path, but I couldn't match the ~, so I went ahead and posted what I had.

Edit

The reason I could not match the ~ in the value of 'rtp' is that when the value is obtained as the value of &rtp rather than the output of :set rtp?, the ~ is expanded to the full path name of the user's home directory. There is no ~ in the result.

The following works.

vim --cmd 'let &rtp = substitute(&rtp, $HOME."/\.vim", "alternate", "g")'

In the .vimrc you choose, you can specify the runtime path:

let &runtimepath=/path/to/specific/vim/folder

If you are using your person .vimrc then you could set this to your personal .vim directory and it should work both as su and your normal account.