I guess the question is how big a fan of water parks are your family and you? If big time fans, then it will probably be worth going out there one day. Unfortunately, your EPs (from staying at RPR) won't do anything for you at VB, but you can still get in the water park an hour early (if you choose to go.)
I enjoyed visiting VB during my last September visit, but I was there for 7 nights too. Personally, if I were only visiting for 3 days, and given you will be lounging with the kids on your arrival day out by the pool. I'd probably save the cost and not do VB on this trip. If your kids would like a few "water" rides, I'd suggest one of the mornings/afternoons you wear swimwear and flip-flops to IOA park. Then with your EPs, first hit the Jurassic park water ride, then walk over to Ripsaw Falls and then flip back and forth between it and Popeye's & Bluto's Barge. You will be very very wet after a few times back and forth. And with EPs you normally won't have to wait very long in either line. My kids enjoy doing that for a couple of hours. We then head back to the room to get dried off and cleaned-up for either lunch or dinner (depending on the time.) I normally bring an extra dry shirt and change that after we finish. That way at least I'm a little bit drier when heading back.