Similar in meaning, but not perfect antonyms:
* Occupy (6 letters) - This means to take possession of a place, the opposite of leaving it.
* Remain (6 letters) - This means to stay in a place, as opposed to leaving it.
Near antonyms with different meanings:
* Retain (6 letters) - This implies keeping something, which can be a way of staying in a place.
It's important to remember that "vacate" has specific connotations of leaving a place, often with the implication of leaving it empty. The above words don't always perfectly reflect this.
Let me know if you can give me more context about how you're using the word "vacate," and I might be able to suggest a better alternative.