In trying to harmonize environments between a ZSH session and the vim/macvim access of the shell, a which ruby from the shell shows $HOME/.rvm/rubies/ruby-1
which ruby
Maybe :$PATH=$HOME.'/.rvm/rubies/ruby-1.9.3/bin/ruby:'.$PATH in your vimrc. Probably hacky, but maybe it'll work.
:$PATH=$HOME.'/.rvm/rubies/ruby-1.9.3/bin/ruby:'.$PATH