A foil is a pair of characters who offset each other through their similarities and/or differences. Poe was quite fond of the foil, especially in its extreme form, the double. In many of his tales the doubling and foiling is much more overt than here. Here it's subtle. The commonalties and differences shared by ordinary foils aren't obviously present in these two characters. The bizarreness of the information we are given resists interpretation. Still, the narrator and the old man (and his eye) are locked in a bizarre, almost mechanized, terrifying dance.
To show you what we mean, we offer this foil moment:
[A] simple dim ray, like the thread of the spider, shot from out the crevice and fell full upon the vulture eye. It was open – wide, wide open – and I grew furious as I gazed upon it. (6)
This of course is followed by the old man's fear. In the line above, both men are reduced to eyes connected by the beam of light. This strange connection provokes fear in the old man, and fury in the narrator. The narrator controls the connection; the old man is subject to it. This is some multidimensional foiling.