My best friend worked for British Airways and all that they ever did in terms of resolution was a voucher for a certain amount. the amount of the voucher varies by amount of original purchase and only for tickets purchased directly via ba.com or a registered travel agent. If the issue has anything to do with tickets that were purchased through a third party site then they will refer you to them.
It sounds like Delta feels they are making a good will gesture. They are giving you vouchers for tickets that were purchased from them at steep discounts and resold to you for less then the ticket via delta (most likely).
You have two options, take the vouchers, since I doubt that Delta is going to offer anything else and if you keep pushing it may decide they don't want you as a passenger (I've seen it happen), or don't take them and don't fly Delta anymore.
There are lots of airlines to choose from.