Hi
Virtuemart and K2Store are two completely different e-commerce components and they don't have any relationship between them. You can use one or even the two on same website. It's user choice. It's your choice
If you want to use VM and make use also of the features that K2 provides, there's a specific component that makes the "bridge" between these to components. I'm referring to
K2Mart developed by the same K2 team.
Cheers