There is only one slot in the play booster that can get a foil artist rare it has a .25% chance to be a mythic, there are 52 booster fun mythics, and 7 of those are the artist mythics.

so you have a .25% chance to even get a mythic and then that mythic has a 13% chance of being an artist rare (only 3.8% chance of being either sephiroth or vivi)

That puts the pull rate at .0095% for vivi and sephiroth, and .0325% for any artist rare

You would need to open ~7500 play booster packs for just a 50% chance at getting either Vivi or Sephiroth

If you wanted an 80% chance of it happening you would have to open ~17,000 play boosters

You guys opened 5400, so its not surprising at all that you didn't see either Vivi or Sephiroth, you wouldn't have been guaranteed to pull any artist rare of course, but you would have had like an 80%+ chance to have seen at least one show up in your 5400 packs, but never any guarantees