When it comes to describing a vapid expression, there are a lot of different synonyms to choose from. Some of the most common include empty, flat, lifeless, dull, bland, and insipid. Each of these words conveys a sense of emptiness and lack of depth, which is often associated with a vapid expression. Other synonyms that might be used to describe this kind of expression include vacant, blank, uninterested, disinterested, and indifferent. Ultimately, the choice of synonym will depend on the specific context in which the expression is being used, and the tone and emphasis that the speaker or writer wishes to convey.