On this page of our guide to Greedfall, you will find a detailed description of Vinbarr's Sanctuary main quest.
Run through the caves and look for traces of Vinbarr. Along the way, you will encounter some battles with monsters - but eventually you will reach the end of the ruins and be transported to Meinei Flag. Upon further walk, you will be confronted by Cera - defeat the woman, and search her body - you will lose 3 reputation points with the Natives.
Eventually, you will reach the passage leading to King Vinbarr. Approach the altars and put the seeds in them.
When you put the seeds in the right places at the altars, the mission is completed. You will get 2000 XP and you will start the quest The Sky's Teeth.