The current vault icon is misleading, I think. It’s more looking like an exit (or entry) icon than a symbol for a vault.
Hence I suggest to use for example this icon instead: https://icons8.com/icon/112580/safe
Here’s a mock-up of how that might look (sorry for the not accurate line color of the icon):
After lightening this icon it looks pretty good. I only have access to the raster version, SVG may look a little better.
With the risk of being called a thread hijacker - the New folder icon is also misleading: 
This icon is commonly used for Open file/folder.
The new file and new folder icons would both benefit from having a “+” incorporated.
