morrow: It uses the vBulletin default function to find similar threads. In fact I didn't even touch the function because it was working just right.
The function gives each similar thread a number, the higher the number the higher the similarities. You can't control it directly but there are some vBulletin settings that might help you.
They're hidden under the search settings. It's basically everything that has to do with the similar threads but I can't give you the exact settings because I don't have an english vBulletin.