It wouldn't kill the Vita or the 3DS for the simple fact that a 7" tablet is big and we don't know yet if it will feature actual controls ala the Wii U's game pad or just a touch screen. Definitely cool though and I'll get one when it's released, I love being an early adopter and all Let's hope this gets Microsoft to release a Bluetooth Xbox controller so we can use it on our Surface too!
A BT controler would be great. I don't know how big the Vita is in the US, but here in Europe I havnt seen them around, and hardly anyone has the 3DS. Something similar to the Wii U would be nice. maybe have it integrate with the 720 in the same way, but allow you to take the games with you instead of needing to be within a certain distance.