I was unable to determine the root cause for the weird VXPD/302 errors and opted instead to provision a new 6.5 vCenter appliance and manually migrating over the ESXi hosts. This required a bit more work (initial setup of vCenter, replication of permission/folders/etc, temporarily breaking HA) but as we do not have any distributed switches or DRS/resource pools, manual migration was the best route the get the issue resolved.
Thanks for you help