The general instruction when using a silicon detector is to either apply voltage only in atmospheric pressure or in high vacuum. Not in between!
I can't find a physical answer to it. Why is it so important?
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I just realized what the problem is. It actually doesn't have anything to do with the detector. When working in vacuum systems you have to worry about the dielectric breakdown of the air as the pressure is reduced. It turns out that the breakdown voltage hits a minimum around $\sim 1$ Torr depending on the species of the gas (see the curves below). This phenomenon is known as Paschen's Law; you can read more about in this Wikipedia article.
If you are applying high voltage across your Si detector when this breakdown occurs, you will surely ruin your detector. It looks like you will be OK at $0.1$ Torr, but I make no guarantees :).