This action card is very confusing. What mission does it refer to? Is the capture of the Leader who rescues a Leader just auto captured? Does the imp leader (vader) need to oppose the mission that frees a guy in order to play this or how does it work?

It's the Luke/Bespin scenario.

1) Rebels attempt a mission to rescue a leader. Let's say Luke rescuing Han.

2) Vader opposes the mission (or is already in the system)

3) The rescue is successful. Han is free.

4) Vader immediately plays It Is Your Destiny. Luke is captured. Better hope the Millennium Falcon is near!

Because Vader is shown on the Action card, he needs to be in the system in order for you to use it. Vader may not oppose the rescue mission, because if memory serves they're often Intel missions and he doesn't have any of those symbols; however he still needs to be present in order to use that Action card.

A leader can still oppose without having the matching icons. The leader attempting the mission would then need to roll at least one success. So you could oppose with Vader, knowing the rescue is likely to succeed, with the intention of playing Destiny.

Because Vader is shown on the Action card, he needs to be in the system in order for you to use it. Vader may not oppose the rescue mission, because if memory serves they're often Intel missions and he doesn't have any of those symbols; however he still needs to be present in order to use that Action card.

You can still oppose even if you don't have the proper icons, there is no restriction on actually having the proper icon. All that happens is that you roll zero dice versus whatever the other side rolls.

And since the action card requires the rescue to be successful...

So be careful of who you send on rescue missions.