If I have the Core Box, GM's Toolkit and GM's Vault what would be best to purchase next if I am reading it to GM?

Depends on what you're planning to do.
As far as GMing goes, you're pretty much set I would say.

Do you plan on designing your own campaign and/or adventures, then I would go with the creature-vault
Do you plan on running mostly official scenarios, then look into on of the published adventures first. (Witch's Song is nice for this)
(The adventures contain all standups & cards & chits you need to play)

Winds of Magic / Signs of Faith / Omens of War / Black Fire Pass really only enlarge specific areas of the game, so you might want to first decide on where you want to go with your warhammer group. If you're not into dwarfs, then Black Fire Pass should not be on your list, if you want to play with commoners and leave out the knights & war-like aspect, then Omens of War is not for you.

It's a bit dated but:

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Nisses offers some good advice above. My personal recommendation would be for the Creature Vault and Guide as having your NPCs and monster stats on cards is simply one of the most useful ideas that FFG has developed for this game. WAY more user-friendly than having a book open or having a photocopy page laying out on the table.

The rest really just depends upon where you and your players plan to take a campaign.

Winds of Magic is all about the Wizard career and has lots of good expansion upon magic but won't be much use if your game isn't going that direction. It contains Corruption and Mutation rules but that is a "fringe" element that you can do with or without depending again on preference.

Signs of Faith does the same for Priests but it also introduces the Disease rules which, while not necessary, add an extra dimension to the "conditions" that PCs can suffer in the game.

Omens of War is all about extra advanced "military" careers and just more options for combat (fighting styles, mounted combat rules, etc). If you're PCs are gonna be martially-inclined, it will be useful but otherwise you can safely skip it.

In addition, the sets above will give you a hearty bestiary of Tzeentch, Nurgle and Khorne daemons and followers, respectively.

Black Fire Pass expands the options for Dwarf characters. Again, if you're not interested in or not going to have Dwarfs in your party, you can skip it and not miss much.

Thank you all for the great advise! I like the sandbox! I am reading the rules currently and will try to get some players once I can teach it. I think I am going to start with some adventures so I can see how they run along with the core adventure. Once I get used to this I will try my hand at some of my own. Is there a better first adventure that another?

I was going to post a thread about where to start if you're interested in the game, but this has answered some of my questions. I still don't quite understand the difference between the big box and the individual rulebooks and accompanying vaults. What's the best bang for your buck if you're looking to try the game and GM a demo for friends to see if it works for your group?

Core set. It has everything you need to play and is available on amazon for around $65.

.......and, its the most cost effective method of obtaining the dice.

Yea, the core set. I just bought it a week ago and it really is a great product. It has everything where the books only have the rules. The books will be nice for the future but the box is the way to go.

Just bought the core box and was going to ask the same question myself. Love the little diagram - this thread should be made a sticky!!

I had my eye on the Adventurer's Toolkit as my first purchase, to add some more career options and expand number of player bits available. And I quite like the idea of the Creature Vault rather than the Creature Guide, if that's a viable option.