1. 徒劳的;无效的;不成功的
    A vain attempt or action is one that fails to achieve what was intended.

    e.g. The drafting committee worked through the night in a vain attempt to finish on schedule...
           起草委员会为按计划完成草案已通宵加班，但结果仍是徒劳。
    e.g. I was singing in a vain effort to cheer him up.
           我唱歌试图让他高兴起来，结果是白费力气。

2. 徒然的;无望的
    If you describe a hope that something will happen as a vain hope, you mean that there is no chance of it happening.

    e.g. He married his fourth wife, Susan, in the vain hope that she would improve his health.
           他娶了第四个老婆苏珊，徒然希望她能使他健康状况好转。

3. 自负的;虚荣的;自命不凡的
    If you describe someone as vain, you are critical of their extreme pride in their own beauty, intelligence, or other good qualities.

    e.g. I think he is shallow, vain and untrustworthy.
           我认为他浅薄虚荣，不可信任。

4. 徒劳;白费力气
    If you do something in vain, you do not succeed in achieving what you intend.

    e.g. He stopped at the door, waiting in vain for her to acknowledge his presence...
           他在门口停住，徒然地等待她和自己打招呼。
    e.g. It became obvious that all her complaints were in vain.
           很明显她所有的抱怨都是白费口舌。

5. 无用的;无意义的;无价值的
    If you say that something such as someone's death, suffering, or effort was in vain, you mean that it was useless because it did not achieve anything.

vain的翻译

    e.g. He wants the world to know his son did not die in vain.
           他想让全世界都知道，他的儿子不是白白死去的。