Im not sure the Ursine are Vastayan. For one thing, they are explicitly given their own name, and they live far from Ionia. It makes sense for
and
(both Vastayans) to live far away, given that they are individuals, and each have their own agenda, but I dont see an entire race migrating away. I think the Ursine are just another race, like the trolls or
's race.
Another thing to consider, is that the Ursine's magic seems very different from Ionian magic. They are mystics, and use the power of thunder, something that we haven't really seen that much in Ionia.
I think that what the lore means by Magic being rarer, is that there are not nearly as many mages as their once were (at least in Valoran, IDK about Ionia), but the few mages that there are are extremely powerful.
is a good example of this. It might also be because of
hiding a lot of world runes, which might have been indirect causes of all the magic before.
Magic has also evolved greatly in Runeterra, especially in recent years. Hextech is pretty much Crystal magic (the kind used by
and the structures on SR.) harnessed by people with no knowledge of magic. Non-hextech technology, as rare as it is, is also pretty much magic. The things that
,
and
are all essentially magical.