If something is vacant, it is not being used by anyone.
Halfway down the bus was a vacant seat.
在这辆公共汽车中部有个空座。
If a job or position is vacant, no one is doing it or in it at present, and people can apply for it.
The position of chairman has been vacant for some time.
主席的职位已经空缺一段时间了。
A vacant look or expression is one that suggests that someone does not understand something or that they are not thinking about anything in particular.
She had a kind of vacant look on her face.
她一脸茫然。