As far as I understand the disappearing bit is done with rotoscoping. You'd have to figure out which parts to mask out first so it looks accurate. Imagining it like an object being submerged in water might help visualize the mask. You know - how water flows in from the edges over flatter surfaces..
Making the reflection accurate though.. If I'd do it, I'd film the scene with two cameras at the same time one being for the reflection.
If you have it, a laser level will help. The type that beams a horizontal or vertical line. It will give you a straight line up your actor that matches the contour of his body, it will give you the line to rotoscope.
Clever idea to use a second camera for the mirror. It'll take some practice to get the angle right, but I think that's the best way to do it too.
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i seem to remember seeing something on how they did the stargate gateway effect for the movie... i probably saw it as part of the b-roll on the dvd back in the 90's, so you might try renting that to take a look at what they did.
i do remember that they did use lasers, like michael mentions... they had several of them all pointing towards where the talent walked through gate. that gave them points to do the roto work... the shot i can remember was a closeup of spader's face as he walked into the gate.