Not impossible, but I consider it unlikely. If they move it forward, it would only be because their original estimates for launch were too conservative, and things are moving faster than they anticipated. It's unlikely to be a response to Nvidia, because there is only so much you can do to make a product ship faster. Especially 3 months faster, which is quite a lot.
The thing is, if AMD feels a bit of competitive pressure from GP104, there are other ways that they can respond. They can add game bundles to Polaris, and/or tweak the price of Polaris. Even if GP104 outguns them, doesn't necessarily mean they have to launch VEGA right that second.
That being said, the danger is that a lot of people have postponed GPU purchases until the 14nm generation. If Nvidia is the only one with an enthusiast GPU and AMD only has Polaris 10, a lot of customers will buy Pascal rather than wait for Vega. And once they have bought Pascal, those are lost sales to AMD.