Steve,
It is precisely this overlap that led to the existence and architecture of the current DPV and DPD drafts, namely that a DPD server was simply tapping into the path discovery logic that a DPV server must execute.
Care must taken however as we go forward with high-level requirements to ensure that a DPD server delivers up that path a DPV server would have used. Or are we otherwise comfortable that some ambiguity may emerge? This was an open question that we as DPV and DPD authors were and are grappling with.