I don't have the vaguest idea.

我一点概念出没有。

I haven't the vaguest idea who he is.

我一点儿都不知道他是谁。

I haven't the vaguest idea what she means.

我一点也不知道她是什么意思。

He has not the vaguest notion of what this means.

他连对这是什么意思的模糊概念都没有。

Don't you have the vaguest idea where you left the key?

难道你一点也想不起把钥匙放在什么地方了吗？