Episode 6.5 of Vinland saga is a 150 second scene that could have been in the main anime but was left out, probably because the episodes would become too long. The scene is about a flashblack/nightmare that Thorfinn has about his past when he was part of the invasion of a village. Since the second season has very little action you may want to watch this to add more action to the anime. The animation, voice acting and sound are all the same as in the main anime. Overall it is worth watching if you like the anime and want to experience the entire thing.In my opinion this scene doesn't add anything unique since Thorfinn has multiple nightmares/flashbacks but it is nice extra content. In the end it's only two and a half minutes so even if you don't like it, you won't lose much by watching it.