Simply put,
– VirtueMart : simple, easy, low functionality, paid extensions available, directly glued to Joomla.
– Magento : clean, difficult, high calibre, good enough for enterprise, but heavy and slow, can be costly for management, have to use bridge to link to Joomla.
ref: http://www.chillcreations.com/blog/22-joomla/190-joomla-virtuemart-vs-magento.html