Thanks for your feedback. First of all, we have disabled all the fields that were previously required, except for "country" and "city of residence". Adding additional information did not really stop spammers in their tracks and - as I have recently learned - also raised unforeseen privacy issues.
vBulletin does actually offer a couple of useful modifications to fend off spam, but these mods also tend to lock out a lot of legitimate users. Fine-tuning all lines of defence is a tricky task indeed. As of yesterday, I have re-activated some of these mods, our logs show already over 400 potential spammers barred from signing up.
On top of the bots, you have humans signing up looking perfectly legitimate, who, probably paid by post, then start spamming the fora, placings links in their sigs, posting inconspicious blog entries or subtly wrapping their rubbish into seemingly related posts. My education on these matters continues on a daily basis, too.