Both the Magento and Virtuemart platform uses PHP tools. But features like composer are inbuilt into the Magento framework, whereas it is just for the namesake in Virtuemart.
The composer is meant to manage the PHP dependencies. In the composer.json the developers define dependencies, which are then handled by the composer automatically.
In addition, it can be used to manage Magento extensions.
However, in Joomla, it only serves the purpose of installation, without being part of the framework as in the case of Magento.
Image Source- https://www.gavick.com/documentation/joomla/third-party-extensions/virtuemart/install-virtuemart
Magento has a console utility that saves 1000 of cache and manages them. Whereas, such functionality is missing in Virtuemart.