As per this github issue thread, https://github.com/KhronosGroup/Vulkan-ValidationLayers/issues/2751 vulkan-layers cant compile and depend on vulkan-headers-1.2.177 or newer. vulkan-headers-1.2.176 or older can only be used. The following line under the DEPEND= section seems to be the culprite. >=dev-util/vulkan-headers-${PV}
These change constantly with live ebuilds and are expected. Eventually it'll hard-require >=1.2.177 and so on, I doubt maintainers want to keep this updated (taking the liberty to close but assigning anyway in case). If you want to pin your vulkan-headers-9999 to an earlier commit rather than current master, you can use EGIT_OVERRIDE_COMMIT_KHRONOSGROUP_VULKAN_HEADERS=... to effectively make it be 1.2.176.