Will this work?

Vulnerabillity Buff

It should.

Think so.

Um, ackshually no

Doesn't work? Can you explain? Increased curse effect does not apply to curse from flask?

From Aura wiki:

"

Blasphemy auras are affected by both Curse and Aura Area of Effect increases.

So is it different to blasphemy aura for some reason?

Never mind, I see. Mona is correct, I should of just checked the wiki for the item.

"

This curse aura's stats are affected by #% increased Effect of your Curses, but not by Flasks applied to you have #% increased Effect[1] or stats that only apply to skills, like % increased Area of Effect or % increased Vulnerability Curse Effect.

Sounds like the generic curse effect would work, but not specifically vulnerability curse effect.

My next question is does increased vulnerability curse effect work for
vulnerability blasphemy? I'm guessing yes, because it is from a skill, but the aura from the flask does not count as coming from a skill. Is that correct Mona?

https://www.poewiki.net/wiki/Replica_Witchfire_Brew
https://www.poewiki.net/wiki/Aura

"

This curse aura's stats are affected by #% increased Effect of your Curses, but not by Flasks applied to you have #% increased Effect[1] or stats that only apply to skills, like % increased Area of Effect or % increased Vulnerability Curse Effect.

Well noted, I have used WFB before and assumed it worked the way I thought as per the OP.

I knew that curse effect stats that specify a curse name are choosy (can only apply to skills), for example they don't affect hex on hit ring/gloves mods.
So I tested this in-game with original Witchfire Brew and a helmet with despair effect. The aura provides -27% to chaos resistance* with and without the helm (unless the resistance icons lie).

I think Witchfire Brew doesn't really "Grants Level 21 Despair Curse Aura during Effect" as it says. Because that would mean it gives you the curse aura skill temporary, but in the character panel this skill doesn't show up. Maybe that's why those choosy curse effect stats don't apply to it.

*level 21 Despair provides -30% to chaos resistance, so should the curse aura from the flask. I've seen a bug report claiming it's actually -25% (tested on Maven), but my tests on a red "resistant" map (25 chaos res from the mod + 8% increased effect from the atlas tree) show it's -27%. Not sure why the results differ, but in either case the flask provides some other Despair, not the one we use.

Ok thanks, good to know.