"I found plugins for that (e.g., buffergrep), but I'll be surprised if this simple task is not natively supported with a vim trick.. is it?"
Not that I know of. And existence of multiple plugins trying to offer this functionality tends to confirm that. . .
What plugins have you tried and what have they been lacking?
http://www.vim.org/scripts/script.php?script_id=2545
http://www.vim.org/scripts/script.php?script_id=2255
Also, just to make sure, you are aware of vimgrep, right? Vimgrep is an internal command that loads files into buffers and does greps on the buffers, with results in quickfix window. I haven't confirmed, but I assume if a searched file is already open in a buffer that Vimgrep doesn't reload it, at least not if it has 'nomodified' flag set. If so, one way to use Vimgrep for quick-and-easy buffer grepping would be to just create a file list for Vimgrep using the output from the :buffers command.