The proc rate of the 2 piece bonus is definitely under 5%... I've noticed it procced 3 times in an evening of karazhan, I wasn't paying much attention though.

So let's say it's a 5% proc rate (being generous), that means it's on average 6 mana per cast you're saving... not worth it.

It's pants. For the love of PopEye it's finally pants.

Is it possible for the +120mana to crit?

Dunno why people complain about set bonuses. I mean, you're getting something for nothing. You're just wearing several pieces of gear that match. Be thankful there's any bonus at all.

Like the old saying goes, never look a gift horse in the mouth.

Can someone explain whats needed to get this item from the vendor? It would be very much appreciated

You have to get a the appropriate token drop from bosses in Karazhan, Gruul's Lair and Magtheridon's Lair. Hand it in at a vendor and get your set item.

... of the Fallen Defender is the name of the token needed for Druids, Priests & Warriors. You will be competing with these classes when you roll for it.

Head: (Helm of the Fallen Defender): Prince Malchezaar, Karazhan

Shoulders: (Pauldrons of the Fallen Defender): High King Maulgar, Gruul's Lair

Chest: (Chestguard of the Fallen Defender): Magtheridon, Magtheridon's Lair

Hands: (Gloves of the Fallen Defender): The Curator, Karazhan

Legs: (Leggings of the Fallen Defender): Gruul the Dragonkiller, Gruul's Lair