The animal was first spotted Monday morning in a wooded area that borders the airport's runway in the central city of Hue. Nearly 100 police officers, soldiers and rangers were mobilized to stop the bull from crossing onto the runway.
On Tuesday, they were aiming to tranquilize the bull.
Airport director Do Chi Thanh says they closed Phu Bai Airport Tuesday morning until 6pm (1100 GMT) for safety reasons.
He says it could be opened earlier if the bull is put under control. The closure could affect 15 domestic flights.
Thanh says bulls haven't been spotted in the area before and it's unclear where the bull came from.