" Will write text including first match until before the second match:
" :WriteMatchedText first second /path/to/file.ext

function! WriteMatchedText(first, second, file)
  exec "normal /".a:first."\<CR>"
  normal v
  exec "normal /".a:second."\<CR>h"
  normal y
  call writefile(split(@", "\n"), expand(a:file))
endfunction

command! -bar -nargs=+ -complete=file WriteMatchedText call WriteMatchedText(<f-args>)