[kh99]

I haven't tested it, but you could try creating a plugin using hook location newthread_post_start and code like this:

$vbulletin->options['postminchars'] = 200;

I'm not sure this handles every case, and I know it won't stop someone from editing the post after it's posted and making it less than 200 chars, but naybe it's good enough for your purposes.