We're working to provide more choice with PlaidML as well. You can use it to run Keras, ONNX, and TF models on AMD, Intel, and mobile/embedded GPUs, on most operating systems and OpenCL, LLVM, OpenGL, and soon CUDA and Metal, etc. We would welcome contributions for TensorFlow integration, ROCm support, and anything else that widens support to more people.
A couple people internally have looked at Vulkan, it seems like it should work really well but we're not actively working on it. You can get an idea of how Vulkan support would look by looking at the existing HALs:
https://github.com/plaidml/plaidml