So the steps are:
-Talk to not-Io before the successor boss fight and restore all the vestiges she has to offer.
-Beat the boss
-View the memory
-Restore the successor by talking their stone body instead of proceeding though the door.
-repeat for future successor
You'll also want to:
-talk to Io at the bae after this fight and get her blood code: Eos
-go back to where not-Io was and pick up the Eos vestige fragment on the ground. You'll repeat this for all the not-Ios.
-the remaining Eos fragments will be found in the games final area.
If you restore all the successors and complete the Eos vestiges you'll get the true ending.
Could it be that the first not-Io doesn't drop a Vestige Fragment when you come back? Instead there was a quest NPC. The next two not-Ios did leave a fragment behind.
I get that people prefer the level design in Souls games but I'm really enjoying CV's areas.
The map lets me know that I've fully explored an area before moving on. I also feel like the level design makes the "awful" areas way less awful than in Souls games.
I don't know how to describe it but if an area in Souls games sucks, it sucks hard but here it's a minor inconvenience at worst. The Highs in Souls areas are much higher so it averages out but I've looked back after completing each area and I can't say I've really hated any of the areas so far and I'm about 3/4 of the way through.