I read your message 3 times but still couldnt get the practical usage of your request, sorry!
If I am not wrong, your requested HTML page would be a "reply thread" alternative, wouldnt it? And if so your users should click a link while reading a thread, to access that page just like the REPLY button in the original vbulletin code. So what's the difference and point of cancelling the original reply screen and creating an alternative which will do exactly the same feature?
If your only concern is the "complexity" of the reply screen you can simply make it more simple by disabling vbcode or similes or make a template modification and get yourself the simplest reply screen vbulletin history has ever seen!
Doesnt that work?
Regards,
Logician