A vehicle includes a vehicle roof and a vehicle floor spaced downwardly from the vehicle roof. The vehicle includes a post slidably supported by one of the vehicle floor and the vehicle roof and a tether slidably engaged with the other of the vehicle floor and the vehicle roof. The tether extends from the other of the vehicle floor and the vehicle roof toward the post. The post is slidable relative to the vehicle floor and the vehicle roof from a stowed position to a deployed position. The vehicle includes a support member mounted to the post between the vehicle floor and the vehicle roof. The vehicle includes an airbag supported by the support member. The airbag is inflatable to an inflated position when the post is in the deployed position.