Yes your VRAM is 8GB. This wouldn't be the first time CPU-Z has gotten this information wrong. In fact as I mentioned, only a few days ago someone had exactly the same issue with a different (but new series) graphics card. Rest assured, you have 8GB, and the author of CPU-Z will eventually catch up his software to correctly display the amount of VRAM.
I'm not sure why the other method I had you try, displayed the wrong amount VRAM. I think CPU-Z displays the information about your graphics card via the device ID. So if the author has the wrong information for that device string, he'll have to fix it. However if author is using some built in method to specifically test how much available VRAM there is, the method I suggested (via...