She wasn't Grand Enchanter, and made no claims to be one. She was FIrst Enchanter, and had a Circle of Magi loyal to her.
SHe was still Court Enchanter, though yes Morrigan was stealing some of her thunder by operating as Celene's mystical advisor. Largely due to the eluvian thing.
Vivienne still had power among the nobles, her Court Enchanter position still holds weight as long as Celene remains in power
She says she could be Grand Enchanter because there's no one left to vote...there's also no more circles.
As a first Enchanter what does she bring to the table?Mages for the Inquisition?Nope .
Researches about magic?Nope.
Does she even rescue loyalist mage you can meet at Redcliffe?Nope.The tranquil?Nope.
Her power at court and among the nobles?A couple of rumors giving war table missions.
She can't get in touch with the Empress when the Inquisition needs it .
You can take her to Halamshiral , she will only give you a couple of advices.That's it.She brings no influence when the Inquisitor needs it the most , nothing.
The only move she makes is in her personal quests , (after Halamshiral...) and she gets in touch with the kids of her lover who are powerful .She also uses the Inquisitor to impress them...
Honestly Sera the street urchin does more for the Inquisition than Vivienne, the devs could have used the war table to show Vivienne using her circle , etc...but nope.