An assembly includes a seat bottom and a seatback supported by the seat bottom, the seatback having a top end. The assembly includes a first airbag and a second airbag, each inflatable to an inflated position and supported by the seatback at the top end. The assembly includes a lap belt extending from the first airbag to the second airbag. The assembly includes a first retractor and a second retractor, both supported by the seat bottom and spaced from the seatback. The second retractor is spaced from the first retractor along the lateral axis. The assembly includes a first tether extending from the first retractor and fixed to the lap belt, and a second tether extending from the second retractor and fixed to the lap belt.