a.

1. 爱虚荣的，自负的，炫耀的
   She is vain and extravagant.

   她既爱虚荣又奢侈。

2. 徒然的，无益的
   They made vain attempts to reach the mountaintop.

   他们做了几次想登上山顶的徒然尝试。

3. 【书】空虚的，无意义的，无价值的

adj.

1. having or showing an excessively high opinion of one's appearance or abilities.
2. producing no result; useless.
   ▸having no likelihood of fulfilment:

   a vain boast.