So the Vulkan mode is using quite a bit of your Ram.
Have you tried lowering some of the settings in the game to compensate the high Vram usage? It may steady the performance of Vulkan mode for now.
If you haven't already done so I would update your GPU driver too.
If you are using an Nvidia GPU, I got a notification today to say there was a new update, so it may be worth checking yours too here -
To download the latest Nvidia drivers, follow this link -
http://www.nvidia.com/Download/index.aspx
To download the latest AMD drivers, try this link -
https://support.amd.com/en-us/download
Let me know after trying these steps it improves the performance for you and we will keep investigating this