Vapidity is the noun for being vapid — the quality of being empty, of having no ideas or interest in it. Where vapid describes a thing ("a vapid speech"), vapidity names the emptiness itself ("the vapidity of the speech").

It is used in two slightly different ways. Uncountable, it is the quality: "I was struck by the vapidity of daytime television." Countable, it can mean a particular empty remark — a thing someone said that carried no meaning at all: "He offered a few vapidities about hard work and dreams, and sat down." That second use is often plural, and it is sharp; you are saying the words were hollow.

This is a rare, formal, and quite literary word. It lives in book reviews, essays, and criticism. Almost nobody says it in conversation, and there is no need for you to use it — but you should recognise it when a critic reaches for it, because it is always a judgment, never neutral.

A gentler and far more common alternative is emptiness, or "having nothing to say".