Vistana (Vistana Resort... not Vistana Villages - two separate resorts both called Vistana) is very close to the parks. Magic Kingdom is a bit more complicated because you have to park at the TTC & take the monorail or ferry to the park. I mapquested the directions and it says 8 minutes to Epcot, 11 minutes to DHS, 19 minutes to AK & 13 minutes to MK. These number sound about right, except 19 minutes to AK - it didn't seem that long.
As far as timeshare presentations, we've been encouraged to attend the "owners breakfast" even though technically we aren't owners. My parents gave us the timeshare as a gift but their names are still on it so we have to purchase a "guest certificate" when we make our reservations. We've always politely declined the breakfast saying we'll be in the park every morning. We usually get one voicemail reminding us of our last chance to sign up for the breakfast but that's it.
The only problem I see with you guys staying off-site is that you sometimes split up. I remember reading about Charles either leaving a park early or meeting up with you later. But I suppose the money you could save staying off-site could be used for a cab or two. There are shuttles between the parks & Vistana but they are few & far between and I believe you have to pay.