VDB is always a bit of an auction. It's up to the GA to offer whatever will get people off the plane, though I've never heard of more than $600 in any situation. It is supposed to depend on how badly you are delayed, and if they have more volunteers than they need, they will often take those who are easy to rebook with minimal delays over those who have complex itins and will require an overnight.
If the GA has enough volunteers that will take DBCFREE tickets, then that is the path of least resistance for them and they will often turn down folks insisting on vouchers.
I've gotten everything from DBCFREE with a reroute on another airline, where the new non-stop actually beat my UA itin home (due to connections on UA) to $500+SWU for taking a bump on an international nonstop that involved adding a connection and about a 6 hour delay.
This is one area where the GAs do have a lot of leeway and where "the speech" can work for you.