Continuing my unprompted investigation into Bickle's work:
On page 132 of his 2000 paper with Worley and Bernstein (really page 16 of the article), they note:
"But in cognition, the problem itself sometimes is to entertain a representational
state with appropriate content. Representing the appropriate ‘‘target state’’ is
the whole point of the process. Vector subtraction is inappropriate for these processes
because it requires that the goal (future ‘‘targets’’) already be represented. (In our
neurocomputational model, this representation is the set of values in the Next Target
Nodes.) Hence vector subtraction is unnecessary for this type of cognitive problem.
Whatever computation is supplying the values to the Next Target Nodes is doing the
work." (my emphasis)
I think that vector subtraction may still play a role in "entertain[ing] a representational state with appropriate content," via the process I suggested in the my last post on Bickle. That is: if the vector for moving from point A to B is stored in memory, it can be used as directions from any other known point C to an unknown point or "sub-volume" D. By using the stored instruction A->B from point C, point (or volume) D is discovered or created.
I think this helps answer the implied question in "Whatever computation is supplying the values to the Next Target Nodes is doing the work." Point D can be discovered via the process in the paragraph above, then the vector B->D can be discovered by ordinary vector subtraction.
... I'm on page 16 of the paper, so maybe that idea will be addressed later on.