Will be means the future. Microsoft constantly obsoletes its own APIs as well, so really it is a poor decision for anyone who is not part of a big company to bother with using it.
You will be able to use vulkan on windows systems, so really there will be no point to DirectX any more.
Everyone obsoletes APIs, however D3D sticks around - DX9 was here for a long time, DX11 was announced 7 years ago, DX12 I would expect to have a pretty long life time as well as the primary windows graphics interface.
Yes, you will be able to use Vulkan, just no one knows when.
D3D12 will be here in just over a month.
Vulkan is still MIA when it comes to anything public and was last reported as 'in flux'.
So, you can start developing with an API which works (D3D11), an API which has docs, beta drivers, good performance reports (D3D12) or you can wait around for Vulkan which is being developed by the same group who mismanaged OpenGL for over a decade.
Personally I'd go with D3D, because say what you like about MS they get shit done and with the last 3 graphics APIs they have done well (D3D10 as a good, if not doomed, API) and by all accounts D3D12 is another sound result.
Khronos and the ARB (who went on to form the Khronos group for graphics), on the other hand, have historically not managed very well and are showing worrying signs of lapsing back in to the old habits which plagued OpenGL.