Vacant is mostly about physical space being unoccupied, while vacuous is about a lack of thought or substance. A vacant flat has no tenant; a vacuous comment has no real meaning behind it. Both can describe a blank stare, which is where the two overlap and people mix them up.
The core difference
Vacant means empty or unoccupied, usually of a place; vacuous means intellectually empty, usually of a person's expression or ideas.
- vacant — not occupied or in use: They moved into a vacant house down the street.
- vacant — showing no thought or interest: He gave a vacant stare and said nothing.
- vacuous — empty of meaningful content, or showing a lack of thought or intelligence: The speech was full of vacuous slogans with nothing behind them.
How to tell them apart
Think of vacant first as a property word: a vacant seat, a vacant house, a vacant job. From there it stretches to a person's expression, as in a vacant stare, but it still just means nothing is currently there.
Vacuous never describes a building. It is reserved for minds, talk and writing, and it carries a mild criticism: a vacuous comment sounds like it is saying something but is not. If you can replace the word with empty and are talking about a room or a chair, use vacant; if you are talking about someone's thinking or their words, vacuous is the one that fits.
