If you pass an absolute path to CMAKE_INSTALL_LIBDIR (which is supported by CMake and used among other things by RPM distro packager's cmake macros), you can end up with:

$ cat /usr/lib64/pkgconfig/vulkan.pc
prefix=/usr
exec_prefix=/usr
libdir=${exec_prefix}//usr/lib64
includedir=${prefix}/include

Name: Vulkan-Loader
Description: Vulkan Loader
Version: 1.2.154
Libs: -L${libdir} -lvulkan
Libs.private:  -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc
Cflags: -I${includedir}

Note that the Libs.private are also bogus, I'll open another issue for that.

I made a PR to solve this in the past, but it couldn't be merged: #214

That's a pretty common issue so I'm sure there's a proper way to do this with CMake... but there's so many ways to do things with CMake that I'm not so inclined to give it another go myself, so I open an issue instead.