I scraped together all components from my Celestia installation so I apollogize for any errors, since I can't really test the addon myself without modifying the rest of my files.
Features:
Dawn spacecraft textured model and trajectory using a timeline
Vesta and Ceres models and textures
sampled orbit for Vesta and Ceres during Dawn mission (ssc defined orbit outside this timeframe)
Known issues:
Dawn ends on september 8, 2016. This is the end of data avaliable on HORIZONS. Dawn's orbit will rise again from Ceres for the final orbit phase and most likely stay in orbit for all eternity.
Dawn's orbit around Vesta and Ceres is sampled, so Celestia will render every single orbit, which causes performance issues. You might want to disable orbit rendering for that reason.
I shifted the Ceres texture a bit to fit the mesh, also Vesta's texture has a slightly different projection than the model, so there is a small missalignement.
Sources:
Orbital data: NASA's HORIZONS system: http://ssd.jpl.nasa.gov/horizons.cgi
Dawn mesh and Vesta model: NASA's 3D resources: http://nasa3d.arc.nasa.gov/models
Ceres texture: http://astrogeology.usgs.gov/search and other NASA sources
Vesta texture and Ceres model: http://space.frieger.com/asteroids/
I also used some texture components by John Van Vliet to improve some properties of the textures.
Download link: https://drive.google.com/open?id=0B14NSu0PnA5xODhWZUZzejBjcFU