All information that we can make public about plans for other GPUs is public, that’s all I can say about that.
Vulkan has limitations in how you can write kernels, in practice you can’t currently use pointers for example. But also, GPU vendors will recommend certain platforms for writing production renderers, provide support around that, and various renderers will use it. Choosing a different platform means you will hit more bugs and limitations, have slower or no access to certain features, are not likely to see library support like OSL, etc.
Our strategy for Cycles X is to rely on the GPU vendors to support us and provide APIs that meet our requirements. We want to support as many GPUs as possible, but not at any cost.