The Vision is a hallucination that Trace has while he is suffering the ill effects of the Pathogen. This Vision takes the form of a boss encounter in which Trace himself has become a Variant, and he sees another man in his own body attacking him.
This is an unusual encounter because the game unexpectedly reverses the role of the player. The player will control the monster during this battle and the computer will take over the "Trace" in the room.
The player will be able to move the Vision slowly around the room, and drop small red projectiles from holes in its body. However, this is an unwinnable encounter. Even sitting directly on top of the Trace in the room, you cannot damage him. He will eventually kill you and the story will continue from there.
Since the encounter is unwinnable, there is not much strategy to the battle. If you want to speed things up a little (e.g. in Speedrun Mode), you can stay low just above the Trace that is shooting at you. Otherwise, just play around until you inevitably die and Trace faints.