It seems that some of the features VirtueMart has, are somehow "hidden" for new and even for power users. I think one of these features is "Parent/Child Products with Attributes". It's possible to have single products, which are a variation of a master product. Say you have the master product "T-Shirt", then you can have different products (with different SKU) for the variations "Size M, Color green" and "Size XL, Color red". Advantage: this gives you the full power of stock control.
Head over to the Flash Screencast: "Product Attributes - Creating Product Variations".