I called Simon customer service today and reported that I was having trouble using my VGCs to make purchases at Walmart. He said they are very much aware of the issue and looking into ways to resolve it. He said they're aware its an issue at WM and at some grocery stores. He advised to try swiping multipe times as it seems to be a "random" thing where it will go through eventually after enough tries.
Anyways, I thought that was hopeful that at least Simon is trying to get it fixed - may not be a lost cause forever.