The most common and most likely answer to this crossword clue is the 7 letter word FOPPISH
I've seen this crossword clue in The Guardian
F
O
P
P
I
S
H
Buddy explains!

The straight part
We know that "Vain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vain = foppish.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 FOPPISH mean?
FOPPISH adjective- Like a fop, a man overly concerned with his appearance; vain and showy.