Detect the real user if you changed to root by e.g. using sudo or su and include the users .vimrc so you have your user configuration while using vim as root

realuser.vim

" Name: realuser.vim
" Version: 1.0 
" Author: Max Boesing <max@kriegt.es>
" Summary: Vim plugin to load users own .vimrc if existent and if the current
"          user is root
if ! exists("g:realuser")
    let g:realuser=system('w | grep $(ps w | grep ' . getpid() . ' | head -n1 | awk "{ print \$2 }") | awk "{ print \$1 }"')
    if $USER == 'root'
        let g:vimrc=system('printf /home/%s/.vimrc '. g:realuser)
        if filereadable(g:vimrc)
            exec ":let $HOME = '" . system('printf /home/%s '. g:realuser) . "'"
            exec ":let $LOGNAME = '" . system('printf %s '. g:realuser) . "'"
            exec ":source " . g:vimrc
            finish
        endif
    endif
endif