I did something like that, though it was very low tech. Just filmed the mirror without the actor, then filmed with and composited it together. You have to watch the lighting and shadows.
are you moving the camera or not? if not, you could print up a poster with the mirror view and attach it to the mirror frame
if moving, you could do something like they did in terminator 2 and instead of a mirror, set up a mirror set on the other side of an empty mirror frame