I used to build web sites, forums being a main component of them, and all that I've built with other forum software has been able to search in the way I expect. Whether it come preprogramed to search in that manner, or there is an official or hacked modification to search the way I expect, it has always been possible. It's just a matter of the administrator or moderator willing to implement it. Granted, I have never worked with vBulletin before, but I find it hard to believe that they don’t have a modification for this. And it has always been as simple as literally plugging in the code package, or the addition of a few lines of code in the existing search script page.
optikalefx, I believe you are right in the way the forum searches, I have noticed that. So sometimes it will be the first 5, 12, 16, 20 threads....who knows until you get deep into the 21 to realise it doesn't contain what you need.