when u do something with lot of hard work,labour or trouble and u pretty well know that u will get no results or no progress at all it is called doing things in vein.

teaching julia is in vein; she is an embecile child.

the troops fought valiantly but in vein; the city was captured within 3 hours

in vein we tried to save the dog; the currants were so big that they swept him away

To do something in vain means that you have done it fruitlessly, for nothing. This is what it means for the following:

She loved him in vain.

He studied all night, in vain, because he fell asleep during the test.

But, this is different:

English also has a phrase “in the same vein” which refers to being in the same category, like the branching of a tree.

I like the French Impressionists, and my work along the same vein.

The Pocket and the Pendant is a story along the same vein as Harry Potter.

vain is like you are so self assured of yourself

veins run in our skin