On the matter of reversing vectors, it also occurs to me that the
destructive variant could be modified to accept a source and a target,
rather than taking elements from and modifying the one input vector.

In fact, I'd like to propose a larger change: change the names, add
start+end arguments to the non-destructive one, and change the
destructive one to accept two vectors.  We'd now have:

   (VECTOR-REVERSE-COPY <vector> [<start> [<end>]])
     Just like the current VECTOR-COPY, but accepts start+end arguments
     as well.  A reversing analogue of VECTOR-COPY.

   (VECTOR-REVERSE-COPY! <target> <tstart> <source> [<sstart> [<send>]])
     Copy elements from SOURCE, starting at SSTART and ending at SEND,
     in reverse to TARGET, starting at TSTART.  A reversing analogue of
     VECTOR-COPY!.

Thoughts?