So I got it working after a short bit of research.
You need to download and install the SDK from https://vulkan.lunarg.com/sdk/home#mac.
(Unfortunately the default install location is ~/VulkanSDK/1.3.280.1/ but I'll overlook that for the moment since I don't have a better place to put it on my mac.)
There are some checkboxes to choose while installing. At first I didn't check any of them, set my LIBRARY_PATH and DYLD_LIBRARY_PATH manually. It built but then failed at runtime with VK_ERROR_LAYER_NOT_PRESENT.
Then I reinstalled with all the checkboxes and it seems to work now.
I'll leave it to another user or a later date to figure out which checkbox put things all in the right places, since it wasn't immediately obvious to me.