That one is easy: just enable the “vim-with-x” USE flag and recompile vim:
# echo "app-editors/vim vim-with-x" >> /etc/portage/packages.use # emerge -av vim [ebuild R ] app-editors/vim-7.2 USE="acl gpm nls perl python vim-with-x -bash-completion -cscope -minimal -ruby -vim-pager”