The most common and most likely answer to this crossword clue is the 6 letter word SPACES
I've seen this crossword clue in The Guardian
S
P
A
C
E
S
Buddy explains!

The straight part
We know that "Vacancies" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vacancies = spaces.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 SPACES mean?
SPACES noun- (heading) Of time.
- (heading) Unlimited or generalized extent, physical or otherwise.
- (heading) A bounded or specific extent, physical or otherwise.
- To roam, walk, wander.
- To set some distance apart.
- To insert or utilise spaces in a written text.
- To eject into outer space, usually without a space suit.
- To travel into and through outer space.