The eyes are supposed to be the windows to the soul. The old man's eye has a 'film' across it: the narrator cannot see into the soul of the old man and is afraid of him> Perceiving him as a 'vulture', the narrator is appalled by the eye and kills teh old man.
The imagined beating heart beneath the planks of the floor signify the surging conscience of the murderer who cannot bear the guilt of his murder; finally, he confesses and tells the inspectors where the dead man lies.
The old man's "vulture eye" denotes evil. The vulture is a black (black = evil) bird that consumes only dead creatures. The darkness of the room and the oft-used setting of night also symbolizes evil.