Hi @dfurlong ,
Just trying to get some possible confusion out of the way; the Index is indeed one number (per Vertex). When you plug a single Value into a Vector, then all Vector components get that same Value.
The Offset is what is being Added to the Position, so if you plug the Index into the Offset, then the Value of the Index of each Vertex is being Added to its original X-, Y- and Z-Position. You can see that here:

For instance at Index 4, the new Position becomes (4+(-1.000), 4+1.000, 4+1.000)
As soon as something is plugged in into the Offset, any Values that were there before are no longer being used.