we will be stating at BWV for five nights
CHecking in late sunday
CHecking out early friday.
Can we buy Dining plan for only the three full days we will be there?
We just got back from a 7-night stay with a similar setup spread over 2 reservations, and it worked pretty well. We started with a 6-night package + dining and finished it off with another 1-night room-only reservation. It was not at a DVC resort, but it seems like it should be applicable to your situation.
The only "issue" I had was with the front desk staff. When I checked in the first time, the man told me that I would have to come back on the morning that my 2nd reservation started and check in again. He assured me that I would be able to stay in the same room with no problem. So, I returned on the check-in date for my 2nd reservation, and was told by the woman working that day that, since the hotel was full, I would have to change rooms. It only took a few minutes (and a phone call to her supervisor) to make her realize how silly it was to move someone who was already un-packed and had been settled in the room for 6 nights instead of moving someone who hadn't even arrived yet. Other than that, it all went relatively smoothly.
The only time we use the DDP is during the first half of a split stay. More than a few days on the DDP, and we feel like we are at Walt Eating World instead of WDW.
If you make two reservations so that you can purchase the DDP for only part of your stay, you will have to check out and check in. And you might be checking into a different room because the two reservations cannot be linked. If they are linked, they are one and you would have to get the dining plan for the entire trip.