I've been working with Virtuemart for a number of years now, and have encountered an issue on the latest 3x version that I wanted to share.
It seems that there is difficulty with the default Virtuemart Cart module if you are attempting to use more than just one of them. Either I am missing something, or maybe I am the only person who likes to have ajax updated aspects of the cart available in multiple places on a site! When doing this with the current version, it really just breaks everything and doesn't work as expected. When clicking 'add to cart' only one of however many Cart modules you have set up will update properly. I have researched this, and its mostly due to how the update_cart.js file is written.
I have re-written this in a way that lets you have as many Cart modules set up as you wish. As a longtime developer, and user of this software -- I'd love to help give something back!https://github.com/trevorbicewebdesign/Virtuemart3-Cart---mod_virtuemart_cart
Its nothing fancy, but it works a little better than before. Maybe someone else will find this useful.