vmap <F12> <ESC>`<:let fl=line(".")<CR>`>:let ll=line(".")<CR>:call ToggleComment(fl,ll)<CR>

function! ToggleComment(fl, ll)
  let i=a:fl
  let comment="//"
  while i<=a:ll
    let cl=getline(i)
    if matchstr(cl, "^\s*//\s*.*")==""
      let cl2=comment.cl
          else
      let cl2=substitute(cl, "//", "", "")
    endif
    call setline(i, cl2)
    let i=i+1
  endwhile
endfunction