Hi Karen and Welcome!
I visited Mapungubwe in early April, which I believe is just after the interpretive centre and restaurant opened. I did not notice any shop, but perhaps I wasn't paying proper attention. Anyway, I wouldn't count on being able to get firewood or meat, best to bring what you'll need with. Surprisingly, I found an excellent butcher at a petrol station in Alldays that's on the righthand side as you're heading towards the park (a largish complex with a cafe, liquor shop and more) . The drinking water at the tented camp and at Leokwe is just fine (I didn't stay at Vehmbe, but haven't seen anything about drinking water problems in the park).
I had both breakfast and lunch at the restaurant and can recommend it. It's the only place in the park that has cell reception and I had some urgent internet work to do, so they kindly allowed me to use the restaurant's top deck as my "bush office"

HTH Have a great trip, Mapungubwe is a fascinating place! And while it's far from Vehmbe, don't miss the treetops walkway and hide overlooking the Limpopo, I spent a lot of rewarding time there, it has a magical atmosphere!