I use VS Code with VLS and Builder, but I think the same is true for Anjuta(is this IDE still alive?) For correct operation Vala Language Server all dependencies should be described in the Build file Meson. For the Vala Language Server to work correctly, all the dependencies must be described in the Meson Build file.
Here is an example that I use with the plugin for VS Code:
project('vala app', 'vala', 'c')
dependencies = [
dependency('glib-2.0'),
dependency('gobject-2.0'),
dependency('sqlite3'),
dependency('gee-0.8'),
]
sources = files('valite.vala')
executable('valite', sources, dependencies: dependencies, install: true)