V-bulletin's search function has been brain-damaged forever. I assume that the stupidity is hardwired and unchangable, since I've been using V-bulletin forums since the early 2000s and all of them have been broken in exactly the same ways. The usual culprit is the "words too short or too common" filter, which breaks damned near every search that involves numbers or dimensions. The lack of boolean search, term grouping, and wildcards breaks the searches that the previous filters somehow managed to fail to break.
Google searches with the "site:" term aren't much better. But at least you can search for, say, "58 cm" and get results. Most of which will be useless, and you won't be able to tell that they're useless without clicking on them, because the two lines of text under the hyperlink will all be the same, but at least you'll have the illusion of success.
Every V-bulletin user and forum site owner/admin in the entire history of V-bulletin has hated the search function, for all of the same reasons. V-bulletin's developers have adamantly refused to change anything about it for just as long. There's a reason that most V-b sites I've been a member of have a FAQ line that says "Search sucks. Get over it." Indeed, so often have I seen exactly that wording that I suspect it's included in the default FAQ that gets generated when a V-bulletin site is built.
Bottom line...
You're screwed.
--Shannon