1. 未使用的;未被占用的;空的
    If something is vacant, it is not being used by anyone.

    e.g. Half way down the coach was a vacant seat...
           车厢中部有一个空座。
    e.g. In every major city there are more vacant buildings than there are homeless people.
           各大城市里空置的楼房都比无家可归的人多。

2. (工作或职位）空缺的
    If a job or position is vacant, no one is doing it or in it at present, and people can apply for it.

vacant的反义词

    e.g. A number of senior people were regarded as likely to occupy the now vacant post...
           几位资深人士被认为有望填补现在空缺的职位。
    e.g. The post of chairman has been vacant for some time.
           主席之位已经空缺一段时间了。

3. （神情）茫然的;失神的
    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.

    e.g. She had a kind of vacant look on her face.
           她脸上有种茫然的神情。