Vivian Guide — Mongil

Pull A0 AND F1 to boost low power fire teams. Pull A2 to really boost fire. Pull A4 to push void+ conquest (not just fire). Choose your char stopping point, then pump Sig dupes instead. Sig buffs stronger than char dupe buffs
Vivian has no off-element relevance until A4

At A0F1:
Vivian's base kit is extremely mid. The values of her kit buffs are similar to that of Jiwon, who is typically not considered for wind unless you are playing Esther for stamina. That is to say: 20% fire is pretty underwhelming. The buff to fire melee is pretty good tho.
Also like Jiwon, Vivian's value is mostly derived from her sig. At F1, it gives 25% atk and 45% crit damage to FIRE DPS, which is comparable to the buffs given by Narae's sig and also Melody with a few dupes.
She also does have a pretty good fire affliction uptime that almost lasts through the entirety of maindps field time. This further improves her performance in Fire teams. The uptime is similar to Angel A4 uptime.
The suppression shenanigans in her support skill does appear to be relevant ONLY for extremely frontloaded/off-field assassins like Angel, Benjamin, Yeonhwa. In a team with Yeon and Ben, she actually matches A6 normal Francis in power. In less optimized assassin teams, Vivian's off-element supporting capability most likely falls behind the usual support cast.
A0F1 Vivian contributes enough personal damage and buffs to warrant a BiS slot in low power/low dupe fire teams. Unlike SFrancis and Narae, who remain BiS for their respective elements no matter what power level you're playing at, Vivian does appear to fall behind SFrancis/Narae with sufficient dupes even in fire.
A0 Vivian is also a mid holder of Amon and a terrible holder of gulgak. Kinda like normal francis.

TLDR: A0F1 is pretty good. BiS in Fire if you don't have Narae/SFrancis duped, sidegrades to the classic supports for specific assasin-maindps teams, and downgrades to classic supports for all other off-element teams
SIG IS STRONG. SIG GIVES MORE BUFF THAN SHE DOES

At A1:
Effectively just adds 10% atk. 12->17 sec is an OK boost to buff uptime too. Atk is a commonly saturating stat, especially if you play either francis alongside Vivian, so in practice, this usually translates to 5-7% ish damage boost

At A2:
Effectively just give 5% more crit rate and 40% more fire, which is a lot of fire. The second paragraph does literally nothing. The conversion to suppression duration remains unchanged, meaning the increased suppresssion buff duration/value doesnt even matter since you can't effectively take advantage of it. AND, CHARACTERS DONT SUPPORT SKILL DURING BURST, MEANING the increased suppression duration/value doesn't do anything during bursts either. The whole suppression gimmick is pretty useless LMAO.
Nonetheless, the raw numbers boost to fire damage means this is still a very strong dupe specifically for Fire teams.
One can expect a 30-40% boost in maindps performance going from A0 to A2, marking A2 as a pretty solid stopping point.

At A4:
The best stopping point for spenders and her strongest dupe. At this dupe level, Vivian becomes the definitive BiS for Fire. Run alongside Narae or SFrancis as the other supporter. Because the devs shoehorned in some big def shred, this dupe level can also be used in the same way as Brisshell for off element. You can expect to boost your Void stage clear for all conquest bosses with A4 Vivian. However, unlike Brisshell A4, she requires a monsterling to enable fire shred and allow her to do things. The defshred uptime also isn't as good. Brisshell is the superior defshred support for off-element vois/abyss conquest