I install the virtuemart version 3.0.18.
As I add products to cart, the popup window will show the message＂Product successfully added to Your Shopping Cart＂and two buttuns (buttun value are "contine shopping" and "Proceed to Checkout" ).
How could I change the message descroptions and the buttun label value ?
Look here for how to do this - it is core joomla functionality
https://docs.virtuemart.net/tutorials/33-templating-layouts/107-customization-of-displayed-text.html (https://docs.virtuemart.net/tutorials/33-templating-layouts/107-customization-of-displayed-text.html)
These are the language variables involved in the add to cart popup.
COM_VIRTUEMART_CONTINUE_SHOPPING
COM_VIRTUEMART_CART_SHOW
COM_VIRTUEMART_CART_PRODUCT_ADDED
To adjust the layout - do a template override to padded.php and put it in this location:
YOURSITE\YOURTEMPLATE\html\com_virtuemart\cart\padded.php