The way VM is integrated into Joomla! is not like any other Component and doesn't comply to Joomla's Component Coding Standards. VM uses its own functions for database access, page navigation, search and listings.

This way it's possible to gain a little bit of independency: VM can be used on various versions of Joomla! (1.0.x series and 1.5 series) and Mambo, although the underlying architecture is different.

Also by using old code from phpShop, this little bit of compatiblity can be maintained (so one can integrate extensions written for phpShop).