It's a feel thing. The geese fly where they do because it is less work for them when following. We can talk about it until the cows come home but won't ever feel it unless we are reincarnated as a goose. Because of this we have to trust and assume that they fly where they do because they can feel that the air in the wake of the front goose is making it easier for our trailing goose to fly the distance.

It really comes down to my standing river wave analogy. If the geese all synchronize their wing beats in the right phasing then they will experince some small energy transfer through the vortex from the goose in front. Because of the area the lagging geese are in the benifit to them will be a combination of the tip vortex and the wing downwash.

Also keep in mind that since geese are ornithopters (wing flappers) any tip vortices will hardly be steady. This would be why you see the geese in a V all beating their wings in time with each other but not necessarily in phase. The sligth phase shift in the flapping would be what the geese feel is required to best ride on the energy of the goose in front.