RX500 is your classic GCN arch, iterated over many years
Vega is GCN with some newer features mainly targeted at commercial/prosumer and more compute oriented structure
RX5000 series is the first of the newer RDNA architecture
Vega used it as it was designed around commercial/prosumer. Previous GCN gamer cards didn't use HBM either, apart from Fiji. It's best to just completely ignore Vega's existence in regards to AMD gamer GPUs.