ugh you're right, my first logout test used words that weren't specific enough so it looked like I was getting too many results, but in fact I was getting the results I should have. When I logged out and tested with robocop AND monkey, the results are exactly 1 post. And I can even drop the "and", it still seems to default to giving results with both those terms. And I saw no differences between my logged out search settings and my logged in settings.
Here's what I don't get... if it's a bug, it seems like a major one, and it would be easy to find people reporting it and explaining workarounds/patches, or at least an official page on vbulletin's forums that says "We're working on it". But on those forums I see nothing at all about fixing this issue. A few google results showed people griping about the problem, but they claim there's no fix. Also they're not on the same modern version of vbulletin and they never mentioned the logout trick. I wonder if somehow you're the first to notice that logging out makes it work properly?
Another idea is it's a setting that the forum admin can tweak, and somehow we have this setting backwards from the rest of the world, maybe that's why so few people have reported problems with it.