Vanguard is an early scrolling shooter. You pilot the Vanguard ship through several underground tunnels clogged with enemy ships. Each zone features different enemies and different natural hazards in the walls. Your goal is to reach the City of Mystery and destroy the final boss, Gond.
The Atari 8-bit port appears to be a hack of the Atari 5200 release of the same 1983 year, but only exists as a prototype. Fortunately, years later, someone was able to convert in into the ROM image, thus we are able to have a look on this port.
This page needs more screenshots.
The melodic synthesizer tune of Vanguard's energy theme was taken from the soundtrack of Flash Gordon. In particular, it comes from the song "Vultan's Theme (Attack of the Hawk Men)", composed by Freddie Mercury of Queen.
(Source: Recognizable bars.)
Ripping Atari music is very arduous process so this is beyond the scope of this site.