I think it'll be your regional pattern (In your example, Polar then)
While I can't give one of those common, credible sources like Serebii or Bulbapedia, since they say jack about this, I've been flicking over various other forum sites and stuff. I can't find anything about people saying that they have a specific kind of Vivillion in their Safari, and a couple of the forums;
>All the vivillon you see will be the same.
GameFAQ
>Anyway, I heard Vivillon would still have whatever region pattern you had even in friend safaris, so bleh.
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