For questions of that kind its much mire useful if you post the worksheet. You can do this by chosing "Use advanced editor" at the upper left.
I am confused as of you variabl for integration - z. Chose another name, as z is the parameter of the function Q' and a constant vector as you wrote. Used the way you did it, sigma is called in the integral with a scalar (which has nothing to do with the vector z) and this will make sigma fail, I guess.
It seems to be easier if you setup your functions to deal with a single scalar value and then call it with the vector using vectorizing.