P.M.
I don't know whether you're more interested in killing the virus, or removing the stain. Since norovirus is so extremely virulent, I strongly suggest disinfecting BEFORE you clean. I wouldn't worry too much about leaving a stain, because it will be covered.
I can't tolerate chlorine bleach odors, so I looked up other appropriate disinfectants. Hydrogen peroxide was listed as effective, so I used some chorine-free laundry bleach, a stronger hydrogen peroxide formula than the medicine-cabinet stuff, in a spray bottle. I used that on every surface I might have touched after having a bout of the nasties last month (was exposed while trying to help my mom deal with explosive diarrhea – omg!!!)
The hydro-perox worked beautifully – nobody else got sick. It's a superior disinfectant for multiple uses. I clean my countertops and doorknobs with it any time the flu, norovirus or other contagion is going around.
Be aware than this product is a strong oxygen bleach, and can have a bleaching effect on some fabrics/surfaces. But that oxygen literally burns the viral cooties to death (while breaking itself down harmlessly into oxygen and water). I'd saturate the affected area, and lightly mist the rest of the mattress. Wait several minutes, and then use a hair dryer on any spots that are excessively wet. (Be careful not to breathe the mist as it can damage the surface cells in your lungs.)
Wishing your household good health. This illness is so utterly miserable.