Here's some "hard to find" Virtuemart plugin development documentation that will help in the development of version 2 and version 3 of Virtuemart plugins:
Virtuemart vmPSPlugin.
./M6
./M6
Software related information one might find useful.
For instance, to give your checkout page a different look, just copy components\com_virtuemart\views\cart\tmpl\default.php into ./debug<your template>/html/com_virtuemart/cart/default.php and hack it.
Actually this follows the MVC Joomla! extensions pattern, so one can apply this customization technique to any component.
./M6