Just to add to the tale of woe here. Also can't complete this dungeon today and have never successfully completed it in the past because of the bugged event chain not progressing.
I really like the Wizard's Vault but including known bugged, and therefore unachievable content in the daily rotation makes no sense, but to then make it impossible to choose something else as an alternative to complete the daily achievement just smacks of poor design.
Really hope something is done about this sooner rather than later, as it sours the whole expansion for me.
EDIT - typically, logged in after sending this and managed to join a group that was partway through completing it, so finally got credit for the minidungeon after multiple attempts, but the point still stands, this shouldn't be down to luck, and there should be alternative options available to complete the daily achievement