The Vulkan specification is Open Source. Many Vulkan implementations are not.

Vulkan also isn't built on D3D at all, and only MoltenVK (an open-source implementation for Apple platforms) is built on Metal. (Edit: It appears Mesa also now has KosmicKrisp as a Vulkan translation layer for extremely recent (≤5 years) Mac devices.)

> WebGPU is available as both a C++ (Dawn) and a Rust (WGPU) library.

WebGPU is implemented by both a C++ library (Dawn) and Rust crate (wgpu-rs), but like Vulkan, is itself only a specification. Also I'd still hesitate to even call wgpu-rs an implementation of WebGPU, because it's only based on the WebGPU specification, not actually an exact conforming implementation like Dawn is.

> Vulkan is also not really cross-platform any more than DirectX .

Sure it is: DirectX makes assumptions that it's running on Windows, and uses Windows data structures in its APIs. Vulkan makes no such assumptions, and uses no platform-specific data structures, making it automatically more cross-platform.

Sure, users of DirectX can be cross-platform with the use of translation layers such as Wine or Proton, but the API itself can't be ported natively to other platforms without bringing the Windows baggage with it, while Vulkan can.

Additionally, Vulkan provides a lot more metadata that can be used to adapt to differing situations at runtime, for example the ability to query the graphics devices on the system and select which one to use. DirectX did not have this capability at first, but added it two years after Vulkan did.

Depends on which extensions, things is with Khronos APIs people always forget to mention the extension spaghetti that makes many use cases proprietary to a specific implementation.