I have a South African passport with a "stati Schengen" visa issued by Italy. The visa is type "C" and is valid for MULT entries.
Can I visit Vienna, flying from Italy, with above visa? Will it be valid?
Yes. "Stati Schengen" is Italian for "Schengen States," which means that your visa is valid for the entire Schengen area. Austria is a member of the Schengen area, so your visa is valid there.
MULT means that the visa is valid for multiple entries, but this doesn't matter to your question unless you plan to fly through a country that isn't in the Schengen area.
In fact, if you travel entirely within the Schengen area (for example, if you fly direct from Italy to Austria, or if you transit only through other Schengen states), you should not even see any immigration control or other passport checks, unless you encounter "temporary" controls that have been introduced in response to the refugee crisis or some other event, or if you encounter the "non-systematic" controls that are allowed under the Schengen agreement.
This assumes that the visa's expiration date has not passed, of course, and that you have not exceeded the 90 days of presence you are allowed in the 180-day period ending on the date of travel.