align comment(s) with Align Them All! plugin on Vim.

.vimrc

" return regexp pattern that matches a comment based on commentstring.
function! s:getCommentStringPattern()
    let pattern = escape(&commentstring, '^$[].*\~ ')
    let pattern = substitute(pattern, '%s', '.*', 'g')

    return pattern
endfunction

" align comment(s) based on commentstring.
autocmd vimrc-autocmd BufEnter *
            \ execute 'nnoremap <silent> <Leader>a/ :<C-u>Alignta v/^\s*' . s:getCommentStringPattern() .  ' <-- -r ' . s:getCommentStringPattern() . '<CR>' |
            \ execute 'vnoremap <silent> <Leader>a/ :Alignta v/^\s*' . s:getCommentStringPattern() .  ' <-- -r ' . s:getCommentStringPattern() . '<CR>'