A few times I took domestic VDB, GA gave me whatever they were offering at the time, i.e. free ticket for lower 48 states and upgraded me to domestic first class on a later flight. Which could be a few hours later and not the next available flight. I never asked for it but s/he gave it to me. I then walked over and standby for the next flight to my destination. GA could not figure out why I was upgraded to First class with my Y ticket. S/he then asked me if I was willing to be downgraded back to coach. I said yes and s/he could not understand why I would agree.

In the OP situation, I would do exactly what dawei did, unless I have time to burn. $600 VDB travel credit is not worth it me if I have to fly out the next day in E- middle seat. Many times they offered very enticing VDB, but I had to meet someone at the destination or could not afford the time.