The Belt of Venus happens before sunrise or after sunset, when the sun is below the horizon. As you know, the bluish or violet light by the sun gets scattered more than the red one, therefore when the sun is below the horizon, some blue light may be scattered and arrive to us, that's why Belts of Venus appear Violet.
Then another point is when you ask why violet light can pass through such a thick layer of atmosphere arriving to our eye. Let me try to explain it with an example: if you put a strong violet source (that only emits violet light) close to the horizon you will see it violet, because even if its light is scattered a lot, most of the light can come through. Not all blue light is scattered, it's just scattered way more than red light.
That's why you see the sun red when it is at the horizon, because the sun emits all wavelenghts (all colors), the redder ones are not scattered much, the blue ones are scattered way more and the net effect is that you see it more reddish.