I think that left-alignment would look better, because with center alignment, none of the usernames would line up at all because they're all different lengths.
That said, the left-aligned name right now is slightly misaligned with the image, and this
has been faintly bugging me recently. Example (with box highlights from Chrome inspect element):
I think it would look better if the image and the username text were aligned along the left edge.
EDIT: Oh, I just noticed that you said "centered vertically". Oops, then we might not be talking about the same thing