What you are experiening is the normal face of a disorganised enterprise. It won‘t help you to know that it‘s not deliberate and that they fully intend to honour your voucher, but I know it can be frustrating. Indeed, that disorganised nature is effectively what put you in this situation in the first place. I assume you did get reimbursed for out-of-pocket expenses as well as EU compensation on top of the flight credit voucher?