Magento offers many features that VirtueMart does not, one of them being the possibility to navigate by custom attributes - the so called "Layered Navigation". You can easily recognize it when you see "Shop By".
Compared to Magento, VirtueMart is a child's play, although it might interest you due to its simplicity. Very easy to install, VirtueMart is the best solution when it comes to junior e-commerce administrators. But you might consider the effort to learn some of the Magento features, since it is becoming the best e-commerce platform.
In VirtueMart, product attributes refers to adding some configurable criteria for a product (and optionally - managing stock values for them). Take for example the attributes Size and Color. You have to define them for each of VirtueMart's products, unless you create a parent product and derive other products from it.
In front-end, these attributes ask the buyer to precisely select the desired configuration of Size and Color for a product. Price of that product can change with various combinations.
In Magento, attributes have two uses:
1 - using attributes for configurable products. We can use the same Size and Color attributes to create a configurable product with various Sizes and Colors. (this behaviour is similar to VirtueMart's concept)
2 - using attributes for a custom product field, an informative field, for which we can filter products by (in layered navigation)
Magento creates the delimitation of configurable products from attributes. You can use attributes to create a configurable product, or you can simply use attributes in layered navigation.
