We ordered from them in October, when we stayed at the BWPPI. We got fresh fruit (bananas and pre-sliced apple packs), milk, juice boxes, microwave popcorn, soda, bottled water, snacks...all sorts of stuff.
Don't know for sure about diapers, but I would be shocked if they didn't have them. They may not have a million brands available for delivery, but they should have them.
The delivery came while we were at the park, and we came back and the perishable food was already in our minifridge. We were really impressed (although, that was the hotel, not Vons)! Check with your hotel and see what their policy is. I think most say that if you have alcohol in your order, you have to be there to sign for it. Otherwise, most hotels are used to taking care of deliveries, but check to make sure.
It's a little more expensive than we are used to, but definitely cheaper than convenience stores. We'll do it every time we go....so easy and convenient!