This idea is not so different from the method of drawing a sigil on a piece of tracing paper, and hanging it over your computer monitor, and then fapping to porn.
When you concentrate on watching the porn through the tracing paper, your subconscious absorbs the sigil while your conscious mind is occupied so that you don’t even notice it, and then it is fired when you orgasm
In this case, most people would look at the mirror through the sigil, in the same way people look through window decorations to see what is inside, or you look through your own reflection cast on a window to look outside. The people concentrating on seeing themselves in the mirror would absorb the sigil into their subconscious mind and would commit their power to manifesting whatever the sigil represents.
The effect would be the same as the classic Chaos method of posting sigils everywhere around town, and having random people charge it up when their consciousness notices it. The mirror wouldn’t bounce anything back because it is simply serving as the surface the sigil is drawn on, not as a reflector of energies.
And no, the sigil would not act as an “open vortex” for spirits unless that was the purpose it was designed for.