It's still very early days for DirectX 12 and Vulkan, the main graphics APIs fighting for superiority. It's not quite Blu-ray versus HD-DVD; even if one does pull ahead with developers, the other won't suddenly vanish. A lot of factors can decide the battle, with one of the more interesting being support for mixed hardware, multi-GPU setups. On this front, Microsoft scored points with DirectX 12, but Vulkan has done one better by supporting multi-GPU on all the important platforms, not just Windows 10.
An announcement by the Khronos, the working group that oversees OpenGL and Vulkan and comprised of industry heavyweights including AMD, Valve, Google, Intel and NVIDIA, among many others, has made it clear that the API's multi-GPU functionality will play nice with Windows 7, 8.1 and 10, as well as Linux.
Apparently, the question was a popular one at this year's GDC, so Khronos thought it best to clarify the situation in writing:
The good news is that the Vulkan multi-GPU specification is very definitely NOT tied to Windows 10. It is possible to implement the Vulkan multi-GPU extension on any desktop OS including Windows 7, 8.X and 10 and Linux ... Khronos always strives to make its specifications as cross platform as possible. Of course, what products ship on which OS is up to the implementers of each specification, but Khronos is already aware of vendor plans to ship multi-GPU functionality on platforms other than Windows 10, including Linux.
The rest of the statement is mostly technical, but the gist is that there's nothing really stopping Vulkan's multi-GPU support from working, well, anywhere. Sure, certain operating system features can make it easier to implement, but there are no hard locks in the way.
Putting aside technical limitations, there's no mystery behind Microsoft's decision to restrict DirectX 12 and hence, mixed-vendor multi-GPU support, to Windows 10. But if a developer was on the fence about which API to go with, Khronos has made Vulkan just that much more appealing.