It was discussed many ages ago. It's currently impossible. Even vB's developers and enthusiasts alike haven't been able to create it. I remember reading a thread about this in vB's online forum community; a rare thread at that - since no one seems to continue the train of thought on this idea, but miraculously it did. Well... to a point. It quickly died down and that was the last I saw of the idea. I couldn't even reply as you have to be a registered member in conjunction with owning a vB license [Don't have the money nor the use for it], so I could never urge continuation on the idea.
Btw,
by now there has to be some kind of mod or hack that could be tweaked to what you're proposing but I have yet to find any.
Ah... It appears I have spoken too soon. It does exist. Well I say: FINALLY!
But the fact of the matter and what stands to be your greatest challenge now, is to convince Hal to add the hack.
I've tried to no avail to ask him to reincorporate the hacks he once added on my asking (QuickQuote & QuickEdit) before the upgrade. This was before all personal problems Hal is currently experiencing so there's no need for anyone to bring that up or use that as an validating excuse. Btw, I hope he is back in a good situation by now as I'm always pressed for time these days to find out myself. Much less my past attempts at PMs never got an answer from the head honcho so...
This is the thread, maybe you can put it to better use than I ever could:
vBulletin.org Forum - Kind of post