Susanoo was crying out for a buff for over a year (I mean the awakened form, let's not even mention the preAW one). She has become a potent damage dealer at her time of AW, but the general meta started shifting towards speed & she once again fell behind. Her nuke has a ridiculously low cap considering her additional ability buff would be there exactly to make that nuke even stronger. At the moment, I need a single or two stacks at worst to reach that cap (or 0 if I'm up against the light rag).

Her rebalance made her pretty viable again. Nuke cap increased a bit & can be further increased with her stacks each giving another 50k, reaching 1m+ with max stacks. Her assault buff becomes 25% each stack, nuke CD gets reset when she bursts & her slow nature is also somewhat fixed with DATA buff from self heal. And also gets burst buff on the stacks as it was mentioned above.

Having that said, as the current things go, I'd probably awaken Satan first unless you already reach 50% def debuff somehow. At the moment, debuffs are still quite valuable & the awakening makes her a pretty fast unit.

As for these vigour & whatnots, those eidolons (if you mean the likes of Fleurety) aren't giving you vigour. Vigour is strictly higher HP -> higher damage effect (and yes, it has its own bracket in calculation). Those eidolons give you special atk as your HP decreases. Special atk effect also has its own bracket, different from vigour.
To put it simply, they're bad, don't bother. Not sure whether they give any bonus to you at 100% HP, but not anything that would be worth it.

And srsly, fk this time-out system. I can't write a single goddamn post without having to relogin to this freakin' site. I feel an extreme urge to just post as unregistered instead.