They aren’t really the same. A ‘vacancy’ is an unfilled position, but a ‘job opening’ implies that the company is seeking to fill the vacancy. E.g. There’s a vacancy in my department but there’s also a company-wide ban on hiring. So there aren’t any job openings.

Vacancy is more common in other settings, such as a room that is not taken.

Yes, they mean the same thing in this context. "Opening" is less formal in tone, and is used more frequently.