Let's look at Newegg for example:Regardless, Vega is selling as fast as AMD can apparently make them. So not that much of a showstopper.
https://www.newegg.com/Desktop-Graphics-Cards/SubCategory/ID-48
Newegg is a large retailer and yet it only has 2 Vega models available (down from 5 a few days ago). Compare that with the number of Polaris and Pascal cards available (hundreds of models).
The same goes for Amazon BTW. The number of Polaris and Pascal cards dwarfs the number of Vega cards available, and most available Vega cards just use the reference design.
Looks like AMD is barely making Vega GPUs, so the supply is very limited. That's why "Vega sells as fast as AMD can make them".
AMD is probably losing money with Vega, so the less it is manufactured, the better.