Download the 6.0 RC and try building one of the Vulkan-based examples in qtbase with qmake:

whereas CMake works as expected.

With qmake it seems that the -IC:\VulkanSDK\1.2.131.2\Include argument goes missing. (it should be generated based on the VULKAN_SDK environment variable, which is set to C:\VulkanSDK\1.2.131.2 on my system)

Same goes for the Qt Quick examples that use Vulkan directly (vulkanunderqml, vulkantextureimport): they end up in:

C:\Qt\6.0.0\msvc2019_64\include\QtGui\qvulkanfunctions.h(12): fatal error C1083: Cannot open include file: 'vulkan/vulkan.h': No such file or directory