Hi. I have used many United VDB vouchers and will try to answer your questions in no particular order.

1) The United vouchers, IMO, are much more valuable than credits. These vouchers are essentially good on any domestic United-metal flight that is greater than two weeks out. A ticket such as PIT -> DRO (Durango, CO) is very expensive; the voucher gets it to you for free. Unless you are saving miles for that free trip to Australia, or unless you absolutely have to have first class, the VDBs are a very good deal. (i.e. You won't get upgraded and you won't get miles.) The fare classes are V,W,and S, I believe, but I have found that most flights qualify for this (but they have to be on United metal, not a code-share).

2) The vouchers must be ticketed within 24 hours, or at least have a postmark (or fedex ticket) within 24 hours of making the reservation. I have scheduled other people on them, although, to be honest, I cannot recall if I sent in the tickets or just picked them up at the airport. I live close enough to the Pittsburgh airport that I usually drive down and get the tickets after calling ticketing. It is a pain that these cannot be redeemed electronically.

3) There are NO fees for these VDBs, whether you go to the airport, or mail them in. You can change the dates and cities on the VDB for a $100 change fee (which effectively can extend the life of the voucher for a year).

If this is too much trouble, I'd be happy to take the vouchers off your hands.