@ diomede:

I looked at the 3 entries in the 'other' forum. I would have preferred that the entries were made here though.

Your render does not reflect the environment, so maybe not all the mirror surface is reflective. Besides the camera angle with the mirror is too steep, so maybe it's a camera trick. I should have added that the lens must be perpendicular to the mirror plane to not more than maybe plus or minus 10 degrees.

In the other 2 renders the camera angle with the mirror is also too steep, so we don't know, although the entire surface of the mirror seems to be reflective.

Using a shader technique for a vampire effect might work for an object with simple geometry and simple shader, like a cube with a single color, but for a model like V4 wearing an evening gown the solution would be very hard to use, so it is not a general purpose solution.

I repeat the links I mentioned above for reference: The Lost Boys and the 2:35 of The Fearless Vampire Killers.