Do not install generated vapi files anymore
Vapi files generated from GIR represent the API of a very specific version of a library. The issue is that the API of a library can change, but the installed vapi file does not. (Only manually)
A solution would be to install only the metadata files. On compilation valac would get the bindings from them + the installed GIR of the libraries.
In case that the processing of GIR + metadata takes longer, than with vapi files, there could be a cache for vapi files generated from the GIR files.