You're certainly not out of line to ask for first on the new flight. On domestic flights I usually hear them offer this in addition to the voucher, so you might not even need to ask.

If you get offered a vdb voucher early before the flight departs you might want to stick around the gate area until they are finished with boarding. If the offer goes higher than what they gave you before they are finished then you're also entitled to the higher offer. Personally I've only been given vouchers after the boarding, but I've read other recieved them on grossly oversold flights earlier then left the gate area before the gate agents were finished boarding the flight.