The phrase "empty vessel" means a person who lacks knowledge or understanding. It refers to someone who doesn't have any valuable ideas or opinions, like an empty container that has nothing inside. Full definition
They're just empty vessels for Android Wear, not devices I'm proud to display on my arm. (theverge.com)
Gone are the days of children being seen as empty vessels into which facts can be poured. (globaleducationmagazine.com)
But we are living in a fashion era when brands seem to be regarded as empty vessels that can be filled with an entirely new set of contents at any whim. (businessoffashion.com)