Vlod will possess you again, at which point it's time for a bit of meet and greet, along with some minor distractions. Once you get outside you'll be unable to fast travel, so make your way on foot or horseback to the marker. There'll be a spider ambush halfway through this bit - and Vlod will give you some grief - but loot's loot! Once the cutscene is over, use Aard on the broken wall in the central chamber, climb through the hole, loot the corpse at the bottom to retrieve a key, and then work your way along the tunnel, looting as you go. Let him possess you he'll eventually end up taking the spot as Shani's +1 for the nuptuals. Naturally, nothing in Geralt's life ever goes according to plan, so a few ghosts will spawn which you'll need to be destroyed before Vlod will appear. Next, return to the central room and ignite the votive in the middle of the area. Use your Witcher Sense and examine the tomb in the north-east area of the chamber, equip the Censer and then spread incense in the marked areas. Upon reaching the central chamber you'll be ambushed by some Wraiths, so throw down Yrden and get rid of them fast. Accept her request to accompany her to a wedding, then enter the crypt. After the cutscene has finished, head for the marker and talk to Shani.
Head back to Oxenfurt, enter Shani's clinic, and then go upstairs.
If you're looking for more assistance, our Witcher 3 Hearts of Stone walkthrough can help. This page covers Dead Man's Party, Open Sesame!, The Safecracker and Breaking and Entering.