Update: Vulkan will not be limited to Windows 10 for multi-GPU support, according to an official communique.
The Khronos Group recently revealed a bit more details about its Vulkan API. One of the nice things about this API is that it supports Windows 7 and Windows 8.1, but it appears that some features require Windows 10.
DSO Gaming noticed that the GDC 2017 slides from the Khronos Group reveal that native multi-GPU support for NVIDIA SLI and AMD CrossFire platforms requires WDDM to be set in "linked display adapter" mode. This feature is exclusive to Windows 10 so this seems confirmation that Vulkan's multi-GPU capabilities may not work with older version of Windows.
Now I don’t know whether there will be enough games supporting such a feature, especially since a lot of DX12 games (and I’m using DX12 as an example because more developers are currently using it) are still lacking multi-GPU support, but these few PC gamers with high-end multi-GPU systems will have to use Windows 10 in order to enjoy Vulkan’s multi-GPU support.
PC Perspective has a bit more background details and speculates it means Linked Display Adapter (LDA) configurations will not be supported but that it doesn't rule out Multi-Display Adaptor (MDA) support on Windows 7 and Windows 8.1. But most game developers will probably not bother with doing all the heavy lifting to enable this.
Do note that this only applies to the Windows platform, it has no impact on Linux support for multi-GPU configurations.
On a related note, Cloud Imperium Games director of graphics engineering Ali Brown revealed that Star Citizen will become a Vulkan-only title. The game developer plans to eventually drop DirectX 11 support and has no plans to adopt DirectX 12 because the multi-platform Vulkan API has roughly the same feature set:
Years ago we stated our intention to support DX12, but since the introduction of Vulkan which has the same feature set and performance advantages this seemed a much more logical rendering API to use as it doesn't force our users to upgrade to Windows 10 and opens the door for a single graphics API that could be used on all Windows 7, 8, 10 & Linux. As a result our current intention is to only support Vulkan and eventually drop support for DX11 as this shouldn't effect any of our backers. DX12 would only be considered if we found it gave us a specific and substantial advantage over Vulkan. The API's really aren't that different though, 95% of the work for these APIs is to change the paradigm of the rendering pipeline, which is the same for both APIs.