The most common and most likely answer to this crossword clue is the 4 letter word IDLE
I've seen this crossword clue in The Daily Telegraph
I
D
L
E
More clues leading to the result IDLE
- A bit of welding up is not working
- Indulged in odd places between jobs?
- Lazy leaving line in fish
- Unemployed one gets appropriate payment, nothing less
- Unemployed superstar given a hearing
- Unemployed superstar, by the sound of it
- Some avoid lectures, spend time doing nothing
- Fish around lake, being at a loose end
- Detailed plan to protect pound is futile
- Not working, taking some paid leave
Buddy explains!

The straight part
We know that "Vain/lazy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vain/lazy = idle.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 IDLE mean?
IDLE noun- An idle animation.
- An idle game.
- To spend in idleness; to waste; to consume.
- To lose or spend time doing nothing, or without being employed in business.
- Of an engine: to run at a slow speed, or out of gear; to tick over.
- Empty, vacant.
- Not being used appropriately; not occupied; (of time) with no, no important, or not much activity.
- Not engaged in any occupation or employment; unemployed; inactive; doing nothing in particular.
- Averse to work, labor or employment; lazy; slothful.
- Of no importance; useless; worthless; vain; trifling; thoughtless; silly.
- Light-headed; foolish.