There are a couple different methods depending on what your restore goals are. You can take a virtual machine image, and you can take a snapshot or backup of that while the virtual machine is still writing to that image. But, that's like walking up to a server and pulling the plug, then plugging it back in, hitting the power button and hoping it turns back on. Often times it works, the application may take care of itself and recover.
Another alternative to powering it down, is to use the snapshot utility in the server virtualization software itself. This will quiesce the virtual machine image, freezing it temporarily while it performs the snapshot. Once the snapshot takes place, you can back up the snapshot. That gives you the complete system state of the machine. So, you can actually take that and bring it to a secondary data center. You could bring it to another environment for testing. It's a true point-in-time copy of the virtual machine image.
Check out the entire Virtual server backup FAQ