The Dun Banner
To get the Dun Banner, you'll need to go into the outskirts of Vergen, and follow the walls to reach a rear section of the city. A few of the locals should talk about the dangers of the tomb, if you ask. The heart of it is quite simple though. Just wander inside through the fairly big entrance.
You will probably want to put some specter oil on your silver blade, to make the fights with the wraiths fairly trivial. You shouldn't find more than a few of them. For the most part, just walk down and through the rooms. You can use Aard to knock down most of the walls. Since we don't have the quests tied into the Vergen side, we just need to look for the marker and then knock down the wall in front of it.
When you find the wraith of the Dun Banner inside, you should be able to talk to him. You'll then need to make a decision. If you want, you can just kill him and take the banner. It's better to deceive him, since you'll also be able to get a really good sword out of that path.
If you tell the wraith that you also fought in the Dun Banner, you can use your knowledge of the battle to convince him. He'll lie about various things that happened, and you'll have to answer him properly. Note that if you have Zyvik's beaver cap, you can use that to get one free wrong answer.
Start off by telling him that he's wrong in his account of the battle, and properly name the enemy as Menno Coehoom. Point out that Menno is already dead afterwards. You can then successfully name the two heroes as Vandergrift and Seltkirk, and finish it up by saying that a priest rescued you.
After that, he will accept that you were part of the unit, and allow you to take the sword and the banner found inside the tomb.
With these items, you should be able to successfully head on back to Henselt's camp. Do remember to get the spearhead from the dice poker game though. You do not want to cross the battlefield two more times for no reason.
After that, just repeat the dangerous run across the fog and make your way back to victory.