VirtueMart Version 3.2.12
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To replicate: Open your store in two different browsers and login at the both end with same user. Now add some product in cart from browser1, and add some different product in cart from browser2. Reaching on cart page the same user is having different products in his/her cart at the same time. You can also try removing all products from user cart from browser1 and refresh the cart of in browser2 and you will see that it would still show products added into the cart for that user in the browser2.
As per my understanding, VirtueMart is updating user cart table from the session. So, for browser1 the user session cart is empty that updates the user cart table as empty. At the same time when page in browser2 is refreshed, it has some products in user cart session and so it again fills the user cart table with those products. And all this results in different cart for user.
This could be a flow of approach that how this e-commerce works. Though here we see major brands like - Amazon(world wide), Flipkart(India) who follows the approach of updating session from table, rather than table from session. You can try the same thing with your amazon account and you'll see the difference. There, cart is always shown same for one user at a time.