Hello, hello! The switch for the gate is… behind the purple vines, when you go down the stairs on the small island in the first room of the cave! However! To magically “purify” the vines you need the Purification Infusion (utility)!
In case you do not have the Purification Infusion, which purifies the Void Vines:
0. Get the Traversing Infusion from the “Careful Treasure Hunting” pirate quest!
1. Go to Dothe Swamp, use the Traversing Infusion on the glittering plant to the left and take the Key Item from the treasure chest.
2. Go to Dothe West.
3. Use the Traversing Infusion on the glittering plant and enter the Dothe Cavern.
4. At the end you will find a poisoned fairy. Give her the potion.
5. Accept her quest.
6. Complete her quest and return to her in Dothe Cavernn.
7. After that, go back to the place where you did her quest and help her again.
8. She will give you a key to open the chest in that area.
9. You now have the Purification Infusion! With this utility infusion, you will be able to make the Void Vines (purple big vines) disappear!
The “side” game of Shattered Worlds is very exploration heavy. Take a moment and talk to various people in Dothe. Some drop hints about stuff!