Perhaps a silly question, but from what I've seen it seems to be the best guarded secret ever.

Does anybody know if you need the Guides if you have the Vaults or vice versa? I quite like playing WFRP with all the materials that come with the original set. Easily laid out information, everyone knows where they stand, all the usual pro points that people cite. But nobody seems to know if the Vaults will require the guides to be used.

If this is the case, FFG are about to take a nosedive in my (and quite a few others') books I think.

It's certainly no secret what the vaults and guides contain. There's a couple of previews in the FFG news section and even a FAQ here. I'll try to answer, but since I'm not certain I understand what it is you're asking you'll have to to accept it for what it's worth. Also, note that I'm talking about the Player/GM Guides/Vaults, I don't really know about the Creature guide/vault to be honest.

1. You can play WFRP3e with the guide books only (well you need dice as well), it won't be exactly the same game as the one we're playing now though. The developers have refered to this as WFRP lite, the Player's guide supposedly contains rules on how to play without using any components or cards. For some this is what have been lacking for WFRP3e. Personally I'd probably not want to play the lite version (although I might consider it if playing with large group or if I have to travel with the game).

2. You cannot play WFRP3e with only the vaults. The vaults contain only the components and cards, you wouldn't have access to the rules if you only bought that.

3. The developers have clearly stated that the core set is still the preferred way of starting with WFRP3e and that they will continue to produce it. If you already have the core set you really only need the guides if you want an extra rulebook (you also get some of the rules from the expansions) and you'd only need the vaults if you want more components/standups/cards than what is included in the core set.

If you're talking about the Creature guide/vault, I've actually wondered that myself, but I'm sure we'll find out any day now since there seems to be a fair amount of people who already got those products.