点津：

valid 有效的，有充分根据的。

How long is the ticket valid?

这车票有效期多久？

The contract is valid for six months.

vain 无益的，徒然的，自负的。

He tried in vain to get into contact with the local branch.

他试图与当地分部取得联络,但没有成功。

She made a vain attempt to persuade him to come back.

vacant 空的，未被占用的，空虚的。

He was appointed to the vacant post.

他被委派填补那空缺.

He applied for a vacant position.

vague 不清楚的，含糊的，茫然的。

I haven't the vaguest idea of what they want.

His thought was too vague to be described.

他的思路混混沌沌，难以理清。

答案：

valid, vain, vacant, vague