After finishing the game for the first time, I was looking forward to the darth vader level again, but it seems that there isn't an option for it. Is there a way to play it twice on the same save or must a new save be created?

You must play it on a new save.

Gotta create a new save. Dunno why, seems kinda backward to me.

what they said

Don't forget, if you need the mastery achievements, all of these (minus lightning shield / force lightning) can be done on the part where infinite wookies spawn. I recommend only doing this if you aren't already close on your main save.

And you can get the worst dayshift manager achievement.