Sooner than I thought they would, but exciting to see. There are details leaking out that Nvidia GPU driver 358.66 will add a bunch of Vulkan support in.
The leak is apparently thanks to Microsoft's Windows Update, so we don't know when it will be released, or if it will include Linux support right away.
It's exciting, as personally I didn't think they would come out at all in driver form until next year. Vulkan is extremely important for Linux gaming, so the sooner it is available, the sooner it can be added into the likes of Unity, Unreal Engine and more. Vulkan should solve some long-standing issues Linux games have with OpenGL, but remember it's not some magic pill. It will take time for people to move to it.
It's still very likely that Dota 2 will be the first game to use it, but I will be more interested to see some graphically heavy games use it.
This doesn't necessarily mean Vulkan is done and waiting for a formal release announcement, it could be that Nvidia is just getting ready early, but we can hope it does mean Vulkan is about to be formally released.