Tyrphanax said:
Leia and Vader have a short encounter on some small planet in the Radio Drama where she's on a mercy mission to an Imperial-controlled planet that she narrowly escapes.

The planet is Ralltiir, where she receives the stolen plans. They're brought to her by a soldier wounded in a diversionary attack. She smuggles the soldier into her personal speeder to rescue him.

Vader confronts her before she's able to raise ship and forces her and the Tantive captain Antilles to stay there and await a search party. She escapes the planet by convincing one of Vader's superior officers that his wishes of detaining her would upset her father. Even though that officer agrees with Vader, he prefers gaining favor with her more than upsetting her and her father. He allows her to leave.

To me, it's one of the better parts of the NPR drama because it's all expanded from the movie. This is still back when Vader was a guy in the Empire military, not some universe-controlling all powerful embodiment of evil. He's very much who he was in 1977.

In this portion of the NPR version, he's disrespectful and mean to her, but he still answers to other officers so he doesn't really have the authority to make her stay or to search her ship.

It would be nice if they showed some of this in Rogue One. It would also be a perfect way to end the film and give them a Vader and Leia cameo scene tying this into the first film.

-----edit-----

I should mention that this part of the story does not lead directly to the opening of the first film. Leia goes back to Alderaan for a short while. She spends some time alone contemplating joining the rebellion in earnest after being more of a supporter\sympathizer. The Empire's brutality on Ralltiir bothers her a great deal.

While at home she and her father also host a dinner for the officer who allowed her to leave Ralltiir. By the end of the evening, she and her father are all-in on joining the rebellion and that eventually leads to the opening of the first film. If I had to guess, the Ralltiir and Alderaan parts of the story probably happen just a few days or so before the start of the first film.