There is a bug in Duels HC mode where after you lose 3 games and end your run, the vault opens empty and it is impossible to exit.
No rewards are shown which is quite anticlimatic tbh.
Also the button to exit has bugged out and is entirely gone.
Please fix the game before adding new stuff that breaks it even further.
This happens to me all the time. If you close the Hearthstone client and reopen, then return to Duels, it will be reset and you can open the vault and get your rewards as normal. Technically, you will actually already have your rewards when you log back into the client, but you can view the animation to receive them in Duels at least.