Random Vibration is a statistical result, so there is no such thing as a maximum stress. You can request a 1-sigma stress, a 2-sigma stress, a 3-sigma stress, a 4-sigma stress etc. Each sigma level capturing a higher and higher percentage of the distribution.
You can take stress component results from the Random Vibe at a particular sigma level and add them to the absolute value of the stress component from the Static Structural. I say absolute value because the stress component in Static Structural can be positive or negative, while the stress component from Random Vibe is only positive. You should do some research on this topic yourself as it is not simple.