Complex sentence: The Vega and Astre were discontinued at the end of the 1977 model year, but the Monza, Sunbird, Skyhawk, and Starfire continued through 1980.
Split into simpler sentences: The Vega and Astre were discontinued at the end of the 1977 model year. The Monza, Sunbird, Skyhawk, and Starfire continued through 1980.