NVidia will already know what is AMDs hand and visa versa.
What AMD could be doing is targeting the 1070 with Vega to start with in the same way as the RX480 targets the 1060. With this in mind there is very little NVidia can do about it with another GP102 card.
There is a lot more money in it for AMD to hit 1070 sales than worry about 1080 and Titan sales.
That makes sense. Maybe they know we're only going to see small Vega, in which case "it is what it is" and there's nothing Nvidia can do about it.
And while everyone assumes Nvidia "knows" through partners or moles (!) or whatever, I also find the other option plausible: that they don't know and would rather wait for AMD to show their hand first. I mean, one of two things could happen: AMD will disappoint in which case it's same old same old, OR AMD have pulled a Ryzen-like miracle and not only are they competitive with Pascal but even Volta. Now, in the latter case they'd be avoiding the embarrassment of having a keynote about the awesome 1080ti, only to have AMD come back the next day and 'drop the mic' on them.
Any way you cut it, there was little reason for Nvidia to make a move right now. It's AMD's turn.
I personally think AMD are not going to divulge anything performance-related. We'll get the run-down on all the new tech inside the chip, but nothing in terms of how well it performs in real world terms.