There is no rule for VDB. VDB is a negotiation. If you don't like what DL has on offer, just reject it.

But, do understand that in this situation, it's unlikely that DL has any IDBs and if it had any before the MX, it is saved by the MX. All those people who are stuck at MCI are not due IDB.

You also lost your leverage. What would you have done if DL had told you no voucher + no UG?