Meson's dependency mechanism searches for either a pkg-config file or a Find???.cmake file. Neither of these mechanisms works to find packages within a vcpkg installation. An option for manually resolving external dependencies does not seem to exist either in meson. The only two ports within vcpkg currently using meson do not require any dependencies so the problem obviously never came up and there is example one could look at to learn how it supposed to be done.

This is a significant hurdle for updating GTK+ and the other GNOME libraries. In principle, meson offers everything needed to build them out of the box, but it is not clear at all how this should be integrated in vcpkg.