Cannot install vim plugins with Vundle

Hi All,
I am trying to install Vim plugins (NerdTree, Tagbar, etc) using Vundle package manager.
I downloaded Vundle into $HOME/Documents/.vim/bundle/ and
placed .vimrc in $HOME/Documents/.vim/
What I can figure out from the error messages is that Vundle attempts to install plugins at “$HOME/.vim” directory which is inaccessible (no permission to create $HOME/.vim).
Then I added in .vimrc:
set rtp+=$HOME/Documents/.vim/bundle/Vundle.vim
Still does not work. Then tried without “+=“:
set rtp=$HOME/Documents/.vim/bundle/Vundle.vim
Still the same.
It turns out Vundle always wants to reach out the folder “$HOME/.vim” instead of accessible “$HOME/Documents/.vim”.
I am wondering, if there is a workaround.