Help with Bone Assignments? Maybe Weight Assignment?

I feel like this skirt shouldn't be causing me such a headache. Maybe someone here can shed some light on it. I'm trying to turn the Get Together belted dress into a skirt, the only problem is I can't get the belted part to let things tuck under it. I've tried a bunch of weird stuff. And this is like my third package so I know the UV isn't perfect (it was on the first one, that's easy to fix compared to this headache!) 

The left is my skirt, and the right is what I want it to do- like have shirts tuck under it. 




Here is the package. Please help? :D