OK, well, I've found the problem but I don't know how to fix it.
VantagePoint required an "owner id" in the GPX file.
We already know that VP does not like GCA files because they aren't GC files
So in order to get a GPX file for a GCA cache that VP will like it must be transformed through GSAK.
The problem is that the GCA GPX files do not contain an owner id.
Sometimes GSAK will output an owner id = "0" and other times it doesn't.
The missing owner id causes VP to dislike the file.
I can't get an owner id into GSAK from a GCA GPX file. GSAK ignores the id attribute if I add it to the GCA GPX file.
So there is an issue in GSAK with population of a "" owner id for GCA caches sometimes, an owner id ="0" other times and VP requires something that is not "".
Two possibilities.
You ask ClydeE at the GSAK forums to add a default owner id to ALL caches that don't have one so that VP works.
When you create the GPX file to send to VP, before you load it you open the file in a text editor (like notepad) and replace all id="" with id="0".
As we're talking external software to GCA, there's not much more we can do to help here.
VP is being a bitch, expecting a value that is not mandatory and GSAK seems not to provide a VP valid file by providing an id=""