"Vegetable tanned" leather?

A simple question: how is “vegetable tanned” leather made, and how does the process and the endproduct differ from the (presumably) regular way of doing it? The insoles of my shoes say “vegetable tanned,” sparking this question.

Vegetable tanned leather is tanned using vegetable based products, as opposed to mineral based oils. It yields a harder, less flexible leather better suited to shoes, belts and leather you want to tool (press images into). Here’s some info.

A quick search turned up these. The first describes vegtable vs mineral tanning and the second brain tanning of buckskins.