The "Offender/Defender" prompt is for when you want to execute a move, you have to hit any one of the four buttons before your opponent hits one and reverses it. You have to wait though till the buttons light up, if you hit one early it's automatically reversed. To do the Doomsday Device, you need to do the same thing and wait for the prompt and hit the button. Once your partner lifts him up, you become the illegal man and have to complete the second half of the move, otherwise it's just a glorified Electric Chair Drop. The Vengeance meter is sort of like the hot tag feature from the last SvR game. I still haven't figured out how to use it though.