Forget I-4. Completely. There is absolutely no need to use it at all when you stay at Vistana.
Vistana's main entrances/exits are on SR535. Come out to that road and make a right turn, then make another right a few hundred feet later at SR536. Less than five minutes later you will pass under the arches that signify the beginning of Disney property, and the road changes names from SR536 to Epcot Center Drive. Five minutes after that you will pass the entrance to Epcot, and a minute or two later you will come to the interchange of World Drive, where Epcot Center Drive ends.
Along the way you will pass under I-4 and you will pass a signalized intersection with Buena Vista Drive. Buena Vista Drive is the main east-west road at WDW, so if you are heading to Downtown Disney you would turn right, and if you are heading to Blizzard Beach, Typhoon Lagoon, MGM, AK, any of the Epcot resorts, the ALl-Star resorts, Pop Century, WWOS, or AKL, you turn left.
Vistana is 15 minutes or less from almost anything on Disney property, and maybe 20 minutes from MK.
Side note: at one of Vistana's exits there is a Walgreens. Make as much use of this Walgreens as possible! Buy snacks and water there, get film developed there, download digital pics to CD there; you can even find a few pieces of luggage and various minor travel sundries like sewing kits and luggage tags and sunglasses there. For more extensive grocery shopping, you can go to a nearby Publix (I can post directions if you need them).