The most common and most likely answer to this crossword clue is the 4 letter word FOPS
F
O
P
S
Buddy explains!
The straight part
We know that "Vain men" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vain men = fops.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 FOPS mean?
FOPS noun- A vain man; a dandy.