When will Vulkan be supported for decode and encode?

Decode to VkImage
Encode from VkImage

There is no plan to natively support Vulkan in NVIDIA Video Codec SDK.
You can use Vulkan CUDA interop and NVIDIA Video Codec SDK for such an use case.

There are a few samples in the repository below that demonstrate how to use Vulkan CUDA interop.
GitHub - NVIDIA/cuda-samples: Samples for CUDA Developers which demonstrates features in CUDA Toolkit