Empty vector menu in QGIS 3.0.0

There is already a bug reported for this problem.

In the meantime you can click "Reset to defaults" under Settings > Options > Processing and then restart QGIS. This removes the empty vector/raster menus.

I'm teaching qgis right now and several student have the same problem. The processing plugin deactivate by their self in several laptops. Just go to plugins/manage and install plugins and reactivate it