vim does not have a method to count word frequency by default, but it is easily added in a function.

Add the following to .vimrc:

function! WordFrequency() range
  let all = split(join(getline(a:firstline, a:lastline)), '\A\+')
  let frequencies = {}
  for word in all
    let frequencies[word] = get(frequencies, word, 0) + 1
  endfor
  new
  setlocal buftype=nofile bufhidden=hide noswapfile tabstop=20
  for [key,value] in items(frequencies)
    call append('$', key."\t".value)
  endfor
  sort i
endfunction
command! -range=% WordFrequency <line1>,<line2>call WordFrequency()

Then it can be invoked by entering :WordFrequency while in command mode.