More likely, this will enable existing titles to run on Linux, making it easier to sell Linux machines like Valve's Steam boxes. Meanwhile, Valve can incentivize native ports on their platforms through many means, so I don't think an OS/2 situation is likely. Also, having a good DirectX 12 implementation on top of Vulkan will likely be useful the same way that Vulkan on top of Metal is useful for macOS ports.

Of course... it's still a long and difficult road for Valve here, given how well emulation will need to work to accomplish this.