In what way is Vulkan not portable? It runs on all operating systems (Windows 7+, Linux, Android, and Apple via MoltenVK) and all GPUs (AMD GCN, nVidia Kepler, Intel), and shaders (compute and rendering) are to my knowledge standardized in the portable SPIR-V bytecode.

WGPU is more portable, since it can use not only Vulkan but also other APIs like OpenGL and Direct3D 11, but Vulkan is already very highly portable for almost everyone with a computer modern enough to run anything related to GPU compute.