Well VR has a constant mean so for example in 10 responses, 5 are going to get reinforced, no matter what but these 5 occur in a random order but with RR there is no mean so every single response COULD be reinforced at any given time, there is no limit, like with VR. ..I think I worded the 'equal chance' thing a bit oddly. Sorry about that.
Example: A poker machine with a VR schedule: The behaviour (playing the slots game) is only reinforced when the person wins the game but this chance in winning occurs 3 times in every 500 games played while a poker machine that runs on a RR schedule may present a winning slot ANY time, which means instead of there only being 3 responses out of 500, there would be any, like 400 out of 500, or 1 out of 500 but because it is not limited, each response has a CHANCE on being reinforced.