finaly found that to make it find the Vulkan dynamic library I had to add -Dorg.lwjgl.vulkan.libname=MoltenVK as a jvm argument.
This is not required when lwjgl-vulkan-natives-macos.jar is available in the class/module-path. By default, LWJGL tries to load libMoltenVK.dylib and if that fails, tries libvulkan.1.dylib. The mentioned jar contains libMoltenVK.dylib, so you don't have to download it separately or build it on your own.
Specifying -Dorg.lwjgl.vulkan.libname overrides the above. Using libMoltenVK.dylib from LunarG should work fine. Note that the override can also be an absolute path to the .dylib you want to use.
All works well excepted that I cannot add any validation layers. If I do, I always get the following error :
VK_ERROR_LAYER_NOT_PRESENT: Vulkan layer VK_LAYER_LUNARG_standard_validation is not supported.
Am I missing something ?
From the list of known MoltenVK limitations:
MoltenVK is a Layer-0 driver implementation of Vulkan, and currently does not support the loading of higher level Vulkan Layers.
This may change soon, there has been some progress in porting the validation layers to macOS.