Because there haven't been any format tweaks since 2005, and since Mirri and Oni in particular have been shown to be dangerously strong (which leads to short, unfun games), R&D is announcing several changes to Vanguard Avatars.
The primary goal here is to keep the format fresh and interesting by giving players more viable options that they can use when building decks. We are still not planning on endlessly tweaking avatars until all of them are completely balanced for competitive play.
The current Vanguard abilities for each avatar can always be found here.
Mirri the Cursed
New stats: 7 cards, 15 life (-1 card, -4 life)
Reasoning: Mirri has been warping the Vanguard Standard metagame by driving out almost all other creature decks. On top of that, it's often miserable to play against, and the mirror tends to be determined simply by who goes first.
Oni of the Wild Places
New stats: 7 cards, 18 life (-1 card)
Reasoning: Oni kills a turn or two faster than we would like, either by abusing storm with Empty the Warrens and Haze of Rage or by utilizing giant creatures with avoidable upkeep effects.
New stats: 7 cards, 25 life (-1 card, +3 life)
Reasoning: The combination of the larger hand size and the card selection made Prodigal Sorcerer too good for "porting" Standard decks (i.e. taking a regular Standard deck and just simply adding an avatar). We think that Vanguard is a lot more fun when players are building new and different decks that you wouldn’t see in a regular Standard tournament.
New stats: 7 cards, 23 life (+1 card)
Reasoning: This is a casual-friendly base avatar that never deserved the hand size penalty.
New stats: 8 cards, 22 life (+1 card)
Reasoning: It's nice to have at least one or two of the base avatars be strong enough that new players can be competitive. With the Sorcerer losing a card, it’s a good time to help out the Warchief.
New stats: 7 cards, 21 life (+1 card, -1 life)
Reasoning: The randomness of her ability combined with a hand size handicap has traditionally made Jaya too inconsistent to compete.
Ink-Eyes, Servant of Oni
New stats: 7 cards, 17 life (+1 card)
Reasoning: The original reasoning behind the six card hand size was that the first ability would make up for the missing card. But in a world where many competitive avatars start with eight cards, it has always been too far behind.
New stats: 6 cards, 18 life (+7 life)
Reasoning: Platinum Angel was originally given severely reduced stats because it seemed the ability was powerful and had the potential to be unfun. Those concerns are still there, but the format has evolved enough that we don't see it being a problem.
New stats: 6 cards, 16 life (+1 card, +2 life)
Reasoning: The last change to the Vanguard format was a huge reduction in Hell's Caretaker's power that took it all the way down to five cards. With Yosei and the other Kamigawa dragons out of the Standard environment, this avatar can recover some of what it lost.