I'm looking at the Dark Obsession card that I bought which is part of the Lord Vader's Command. The card description is as follows:

Play this card as side mission "Dark Obsession" (Dark Vader, Lord of the Sith, pack). After the heroes resolve any other side mission discard this card and receive the reward listed below.
Reward: "Darth Vader (Lord of the Sith)" villain.

If I look at the mission itself it says if Imperial wins then they get Darth Vader. Which makes me very confused, does this means you get him whether you win or not its just a matter of when?

No. The next time the heroes choose a side mission they can pick from Dark Obsession or the other two side missions available. If they dont pick Dark Obsession you get Darth Vader after the side mission ends. If they do pick Dark Obsession then you only get him of you win.

Basically, their only chance to block you from getting him is to play the mission at the next available opportunity and win it.

By RAW (Rule As Written), as the Imperial player you cannot bring in villains (the ones with a square in front of their name, ex. Darth Vader, General Weiss) as an open group until you win their respective agenda mission first, winning Dark Obsession simply unlocks that ability

If you lose Dark Obsession then you do not unlock the ability to bring him in as an open group, but he might still show up for free if he is part of the mission's Reserved Group or Open Group (although I don't recall any mission that grants Vader as an Open Group except perhaps Dark Obsession itself)

You could also purchase and then use the As You Wish agenda card without winning Vader as a villain. But you still need to pay for the deployment cost to deploy him.

I see, thank you!