Very old legends state that a mirror doesn't actually reflect you, but what you are actually looking at is your soul. There was actually a fairly common superstition that you shouldn't have a baby near a mirror, since it's so young it might lose its soul for good.
In the context of the supernatural, it was commonly held that witches cast no reflections since they had no souls--their souls were lost to the devil. If a vampire has no reflection in a mirror, then the most likely case is that it has no soul either, since there is nothing for the mirror to show.
As far as cameras go, one could say that the act of taking a photograph captures the image of the soul, rather than the person him/herself. By this logic, vampires, having no souls, would continue to not be captured by photographic devices.
Portraits, on the other hand, are just paint. Nothing supernatural about them, save for that Dorian Gray fellow.
Source: The Sorceror's Companion, Allan Zola Kronzek and Elizabeth Kronzek, p. 148
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