This might be only personal experience but i find it that at a certain lvl of gear the elder vaults become way too easy to plow through… to a point where it becomes tedious duty that might bore the players out of the game.
I suggest a simple solution… use the same mechanic that is used to increase a power of the summoned surge.
Before u enter the dungeon you put the Swirling Chaos inside some altar… increasing the damage and health of mobs inside the vault. That way it can be absolutely optinional to increase the difficulity just to offer some better geared team of players to still feel challenged. As of term of reward you can pump up the chest at the end with more Eldarium.