Although Viggo on Glass only has 20 aircrafts to destroy, General Viggo still mentions 25 in the cutscene. As a result, you can pass on destroying 5 of the aircraft.
In Viggo's Revenge, some of the aircraft have warped textures (mostly the ones within the hangers). It appears they were not properly mapped to the 3D surface.
During the aircraft battle, after downing all enemy aircraft, the player is meant to land the jet back on the launch deck after which a cinematic will play depicting the player launching the fighter jet off the runway (without a pilot) forcing it to crash into the ocean, destroying it. Due to a scripting glitch however, landing is not required. One can merely crash the jet into the deck or ocean (and die/respawn) for it to be counted as destroyed, though this skips the cinematic.
Even after destroying it, the jet on the flight deck can still be flown by simply jumping in the direction where it's open cockpit once was.