BOI Controller described MVA separation event when an incorrect altitude assignment was issued and failed to note the altitude assignment read back.

Narrative

A CRJ900 checked on from the north east of Boise; descending from 15000 to 13000; I gave a decent to 11000 and described the weather in the area; in relation to thunderstorms and precipitation displayed and previously displayed on the RADAR scope. I then gave the CRJ900 an initial vector heading 225; and verified the decent to 110; within a minute I observed them descending below 110 in the 11000 FT MVA; I climbed the CRJ900 back to 11;000; initially believing it may be a pilot deviation. After I was relived from position I went to listen to the tapes before calling the aircraft to discuss this. Upon review of the tapes; I found I had issued a turn and a decent to the pilot on the initial vector with a decent to 100; resulting in the decent below the MVA. I reviewed the strip and the markings all indicated that I had only descended the aircraft to 110; then 080 before being cleared for the approach. Further vigilance of what I was saying and what I was hearing back from the pilots would have prevented this along with an alertness to the decent rate of the aircraft.