Oil/liquid on the harness is most likely transfered over from the vest. The PVC-like material of the harness is unlikely to deteriorate.
The only reason why I wonder if this is not the case because in a few cases with mine I noticed that the harness had oil in places where there was not the slightest bit of oil on the shirt underneath. Like the front bottom of the harness where it was not even touching anything, in fact I wonder if the harness is the problem and not the shirt?