Latest versions of IUnityGraphicsVulkan.h have a new IUnityGraphicsVulkan2 interface to solve this problem:

// Vulkan API hooks
//
// Must be called before kUnityGfxDeviceEventInitialize (preload plugin)
// Unity will call 'func' when initializing the Vulkan API
// The 'getInstanceProcAddr' passed to the callback is the function pointer from the Vulkan Loader
// The function pointer returned from UnityVulkanInitCallback may be a different implementation
// This allows intercepting all Vulkan API calls
// The priority is used to sort multiple callbacks such that the highest priority will be called last
// with the original Vulkan loader implementation of vkGetInstanceProcAddress passed in as 'getInstanceProcAddr'.
// A priority value of `kUnityVulkanInitCallbackMaxPriority` is used to force a callback to be called immediately before
// the original Vulkan loader implementation of `vkGetInstanceProcAddress`. Only one callback can be registered with a
// priority of `kUnityVulkanInitCallbackMaxPriority`, if one already exists it will be replaced.
// Passing a priority value of `kUnityVulkanInitCallbackMaxPriority` is equivalent to calling the `InterceptInitialization` method.
//
// Most rules/restrictions for implementing a Vulkan layer apply
// Returns true on success, false on failure (typically because it is used too late)
bool(UNITY_INTERFACE_API * AddInterceptInitialization)(UnityVulkanInitCallback func, void* userdata, int32_t priority);
// Remove vulkan intercept initialization callback.

// Removal will not take effect until the next time vulkan is initialized.
bool(UNITY_INTERFACE_API * RemoveInterceptInitialization)(UnityVulkanInitCallback func);

This shipped in 2022.1.0a7, 2021.2.0b9, 2020.3.17f1 (~August - September 2021 timeframe)