The definition of PFN_vkVoidFunction is:
// Provided by VK_VERSION_1_0
typedef void (VKAPI_PTR *PFN_vkVoidFunction)(void);
This type is returned from command function pointer queries, and must be cast to an actual command function pointer before use.
For more information, see the Vulkan Specification
This page is extracted from the Vulkan Specification. Fixes and changes should be made to the Specification, not directly.