Vivien the Enchantress

What does 'wherever she is' mean?

Anywhere! The realm, your hand, and cemetery, and even while in the middle of your spellbook.

Since Avatars are Spellcasters, is Vivien a Spellcaster?

Not by default. Avatars don't have the printed ability 'Spellcaster,' so Vivien must gain the Spellcaster ability another way, e.g. a minion with the printed Spellcaster ability.

Can Vivien the Enchantress copy the abilities of another Vivien the Enchantress?

No.

What happens if Vivien uses the Druid's ability to summon Bruin and flip?

She summons Bruin and then flips. As a flipped with nothing on the back, she has no game text and will effectively do nothing for the rest of the game. She is no longer a , has no or abilities, etc. The flipped can exist in , and technically she could turn off , and would be destroyed by .