The most common and most likely answer to this crossword clue is the 2 plus 2 letter word NO GO
N
O
G
O
More clues leading to the result NO GO
Buddy explains!

The straight part
We know that "in vain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in vain = no go.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does NO GO mean?
NO GO noun- (with indefinite article) Something which should not or cannot be done.
- Not functioning properly.
- Of a place, not to be entered.