I think it depends on how it is presented.

If they say simply it is a voucher good for $300 on Delta and don't at a minimum say "with restrictions on use", or simply say "non-transferable, only one voucher per ticket, good for one year", they have fraudulently concealed pertinent facts from the customer.

Given the fraudulent concealment, one should be able to rescind the contract, in which case you would then be entitled to Involuntary DB compensation.

In California (can't speak to other states), small claims courts have the authority to grant rescission.

So, if Delta is asserting terms of the "Delta Dollars" that were not disclosed, one course of action would be to file a small claims court action asking for rescission of the agreement to accept the "Delta Dollars", and for payment of the involuntary denied boarding compensation.