The FMS I fly with typically has dual GPS, tripple IRS, and then the DME-DME update you referred to. I can't imagine the odds of losing all of those simultaneously (not with the many back-up electrics on the jet I fly). So, I consider the VNAV approach acceptable as is, based on my experiences with these systems.
We have dual ADFs installed on our jets. It would be interesting to hear for a fact that there are companies who choose not to outfit their international fleets with at least one ADF. I'm always amazed how far out I can receive the NDB at Prins Christian Sund in Greenland, and Nain NDB north of Goose. Both have been solid in excess of 250nm. I think NDBs provide improved SA when on vectors as well (not to mention a source for news during cruise).