Vienna NT in defense is used after opening on
the 1-level. You bid 1NT that shows:
17 Hcp + Undefined ./ 19 Hcp + NT hand.
I have used this one before and can only say good things about it. The down thing is that you can't bid 1NT with 15-18 Hcp.
You have to double. But hey, you cant get everything in life. The answers on 1NT is:
2C : 0-7 Hcp.
2D : 8-12 Hcp. Game forcing.
2H : 8 Hcp +. 5+heart suit.
2S : 8 Hcp +. 5+Spade suit.
2NT: 13 Hcp +
3C : 8 Hcp +
3D : 8 Hcp +
3H : 5-7 Hcp. 5+heart suit.
3S : 5-7 Hcp. 5+Spades suit.
3NT: 8-12 Hcp. At least 5-5 in minors.
4H : Not to many honors, but very long suit.
4S : Not to many honors, but very long suit.
1NT - 2C
2D: Ask partner to bid his best major.
With 6/7 Hcp. he can jump to the 3-level.
1NT - 2NT
Now you can use 3 club as Baron. This is a conventions where you bid you 4 card suit as low as possible.
When you are going to answer on 1NT you bid your suit even if your opponents has open
in that suit. The reason is that even if they has shown a 4 card suit in spade you still can have game in that suit. Many laugh about this, but I can promise you that it works. (TheOne)