Vegetable tanned leather develops patina and ages richly over time. Environmental factors like light, air, and your skin oils also starts the patina process.
If you are not patient enough for waiting, you can also use some neatsfoot oil for to speed up the process. :)
We use Hermann oak leather for most of our vagetable products. Hermann oak leather company has been making leather since 1881in USA. Their specialty is vegetable tanning.
Here is before-after picture.