Seems hard to believe it is a yield problem with the dies : 3 possibilities:
1) HBM2 supply is still just too slow, Nvidia is taking a most of the stock at inflated prices to sell in GP100/GV100. Or perhaps the bonding is leading to lots of defects, but one would have hoped AMD would have resolved this after Fiji.
2) AMD knows Vega is a bit if a distser and wont really garner much sales. Instead, they are maximizing the output of Polaris dies that are being sold a high margins to miners. Plus Polaris level GPUs are more important to maintain Market share.
3) There is still a hardware/design defect resulting in Vega's massive power draw and lackluster performance. AMD are doing another re-spin so the AIB chip will be late but hopefully faster and cooler.