Vala uses the name of the *.vapi file to determine the libraries to link
against. Since the pa_glib_mainloop_*() functions are in a separate
library (libpulse-mainloop-glib.so) the corresponding objects in the
Vala bindings have to be in a separate *.vapi file.

If you are compiling an app without the GLib integration you could use:
 $ valac --pkg=libpulse test.vala
but if you do use GLib you can use:
 $ valac --pkg=libpulse-mainloop-glib test.vala
(libpulse is a dep of the libpulse-mainloop-glib so no need to specify
it explicitly)