An assembly includes a vehicle roof and a vehicle floor spaced from the vehicle roof. The assembly includes a seatbelt assembly for accommodating a wheelchair. The seatbelt assembly is slidably supported by the vehicle roof and the vehicle floor. The seatbelt assembly is slidable along the vehicle roof and the vehicle floor along a vehicle-longitudinal axis. The seatbelt assembly has a seatbelt webbing elongated in a direction from the vehicle roof to the vehicle floor. The assembly includes a seatbelt buckle slidably supported by the vehicle floor. The seatbelt buckle is slidable relative to the vehicle floor along the vehicle-longitudinal axis.