I saw that tweet but was curious about it...I always thought she never quite got to become the First Enchanter b/c of the mage rebellion.
The same. That's why I was surprised. But The op asked was vivienne meant to become and Mary answered she is. I think it's clear. After that, I read the lore about what is and what does a first enchanter in a circle. I must admit, I never was so interested by " what is the meaning of this job. " 
That's what I found with wiki :
A first enchanter retains their position for life and reserves the right to appoint a successor should the first enchanter perish or choose to retire[2]. This decision must also take into consideration the opinion of the templars. Eligible for this position are the Circle's senior enchanters.[3] If there is no successor, the Knight-Commander appoints one, usually with the approval of the Circle's mages.[4] A first enchanter is usually someone who is respected by all mages and usually the templars too, and he or she is not always chosen based off their power and abilities.
A first enchanter officially has a broad range of powers. Their permission or consent is needed:
- for a mage to leave the tower;
- for templars to summon an apprentice for the Harrowing;
- for templars to make a mage Tranquil;
- for templars to move overtly against a Circle mage suspected of practicing forbidden magic.
However, the extent to which the templars allow a first enchanter to exercise their powers varies from tower to tower.
First enchanters routinely congregate in the city of Cumberland, at their College. There they elect a new Grand Enchanter every few years.
So Vivienne, first enchanter, courtier, advisor to Celene and fearsome for the Orlesian court ? She will be soooo interesting, and It will be sooo interesting to hear her stories !!!!
Otherwise, When I heard the E3 demo, I also thought it was very similar to Brienne of Torth when Vivienne screamed. I'm also thinking it is very likely that Gwendoline Christie is voicing her !