I will say this, that Polaris GPUs "don't" decode Vp1 in hardware either, but they DO work, because AMD implemented a Hybrid decoder, that uses the CPU, and the shader processors. By now, I would assume that AMD implemented the same scheme for Vega-based APUs.
The fact that some knuckle-heads are saying this is a bad thing, somehow, and that video-decode support isn't "truely supported" unless it's contained in a full hardware pipeline (even though Intel has implemented hybrid decoding in the past as well), is truly mind-boggling.