The relative equivalent to :source
is :runtime
.
source ~/.vim/plugin/someplugin.vim
becomes
runtime plugin/someplugin.vim
With this, as long as you also adapt the paths in the 'runtimepath'
option, it should work.
Alternatively, you could also change the value of $HOME
inside Vim; this affects the expansion of ~
, too:
:let $HOME = '/home/yourlogin'