他们寻找丢失的文件，但徒劳无功。

They searched ___in ___vain for the missing document.

我们徒劳地等待着承诺的回复。

We waited ___in ___vain for the promised reply.

他试图说服她的一切努力都白费了。

All his attempts to persuade her were ___in ___vain.