The data below originates from one unnamed AMD partner but is expected to be similar for most other vendors, give or take a a couple of days. It's revealed that the AIB partners will not receive the final BIOS from AMD until August 2nd.
So basically, the custom cards won't be finalized until early August and won't enter volume production until around mid-August. We may see some small volume in August but volume availability of custom-design Radeon RX Vega cards isn't to be expected until sometime in September as the cards travel by ship to minimize shipping cost.