If you open the package manager (user the tool bar button that looks like a cube; top row, 6th from the left), you'll find a "scan hub for missing packages" button at the bottom. This will automatically download any missing assets if they're on the hub. If they're not, then you need to track them down on your own. They're probably from somebody's Patreon page.
If you aren't already, it's a good idea to browse the hub from within vam itself, rather than using a web browser. That will give you the option of downloading everything you need with each scene.