Well I found the answer, though to describe it as badly documented is an understatement!
First of all, this TechEd article highlights the fact that users can “enable or disable any of the four evaluations that are available in symbolic links”. Those four “evaluations” include remote to local and local to remote. It doesn’t give any clue as to how to do this.
However a further search revealed this fsutil help page, which does actually document how to “enable or disable any of the four evaluations that are available in symbolic links”. So to fix the problem I was having, I need to issue the following command on the Vista box:
fsutil behavior set SymlinkEvaluation L2L:1 R2R:1 L2R:1 R2L:1
in order to allow full access to where symlinks are pointing on both local and remote machines.