They have converged to the point that the APIs are extremely similar. To the point that you can add a translation layer from Vulkan to Metal (MoltenVK) and lose very little performance, for example.

The concurrency primitives are very similar. The way they all manage memory is a little different, but not horribly so.

And now that DirectX and Vulkan are both supporting SPIR-V as the shader intermediate format, things really have converged.

Don't forget that AMD's Mantle API is the common ancestor for both Vulkan and D3D12.

I'm not OP, but it's worth noting that at least Vulkan and D3D12 have a common ancestor in the form of AMD's Mantle API.