Change "vector" (lookup-based) access to access via integer index

Change from "vector" indexing (requiring maths) to integer indexing in one more place. Other places calling this method with "vector" indexing have been updated so they don't do so, which is why the case is now redundant.

Actually it just bubbles up the vector lookup to the caller. What we need is to "bubble" it high enough up that we can wrap the whole thing in an object.

Towards #1005.

I had forgotten to push this minor change in the actual sprint, sorry for the oversight.