vbulletin search is fine, indexing can be customised in control panel to anything, two characters is popular choice, default is three and it's a left over from old days when a typical forum setup was a massive, old mysql database running on a 120Mhz Sparc and a drive with IO so slow it had performance times quoted in three digits. For a technical forum 3 letter limit is very unfortunate, just think of all the hardware symbols, abbrevs and company names with three letters you cannot search for, especially since search already has anti abuse timeout, but in the end - in case of a busy forum it does limit load on server.
