Zwei ap Owen's page

Level 16: HP: 108, AC 17. Vital Strike adds a bit of extra damage now. 6d8+84+8d6. Tiger now at 1d8+24/1d6+24/1d6+24/1d6+24/1d6+24

Level 20: HP: 134, AC 17. Improved Vital Strike is now in effect...

Unfortunately, Vital Strike cannot be used as part of a charge

You right; totally misread that spell. So I recant, and say a d20 roll is a d20 roll, and that Ill-Omen can apply to a saving throw forced by Persistent Spell (but requires a second charge of Ill-Omen, so this only works for Witches of level 10 or higher).

Persistent Spell is different because it's a re-roll, which isn't the same as rolling twice. In the hierarchy of events, Persistent Spell only kicks in if a target has made its save, which is a self-contained event that can carry with it its own modifiers. So if a target is effected by an Ill-omen spell cast by a 20th-level Witch, they would first take the lowest of 5 rolls. [I can't read] At that point Ill-Omen's effect ends, since it only applies to one d20 roll. If the target has managed to pass it's saving throw at at that point, then Persistent Spell would kick in and the target would make one more roll (effectively taking the worst of 6 at the end of the day).

Whether Persistent Spell carries with it the benefits of True Archmage (which reads "any saving throws against the spell") etc., is another question, but takes us back to mythic territory.

But if you want a fix that's fair to players who end up with overlapping abilities and that doesn't break the game, I think as you say treating them as multiplying multipliers is probably the most reasonable course. That's just fiat, though; I stand by my legalistic interpretation for anyone who thinks technicality should carry the day.

I have a similar question related to Recalled Blessing and Eldritch Breach (and of course your question is mirrored with RB and Divine Vessel). Both say to roll twice and take the higher value for Caster Level checks to, say, Dispel an effect.

In my opinion and absent any formal ruling on the matter, you should treat any given roll independent of others when determining any ancillary effects on that roll. There doesn't seem to be any meaningful distinction in the rules between 'rolling twice' and 'making two separate attempts', so to speak. So yes, RAW (or not written, as the case may be) I would rule that Recalled Blessing + Eldritch Breach would allow you to roll four times and take the best result when attempting to dispel an effect, and that Arcane Surge + True Archmage forces non-mythic characters to roll four times and take the lowest result.

A corollary of this is that Arcane Surge + True Archmage + Mythic Spell Focus would arguably force a non-mythic target to take the lowest result of 8 rolls (!). There's a very tenuous argument that you could treat this as multiplying multipliers, in which case you'd end up rolling 6 dice instead of 8. Both scenarios sound ridiculous (and kind of are), but hardly seems problematic since the only time they apply would be when a Tier 10 Mythic character is using 2 MP against a non-mythic target, which in all circumstances is like sand-blasting a cracker anyway.

A corollary of the Recalled Blessing + Eldritch Breach scenario would be that even as you roll four dice and take the best, you can only Surge one of them (admittedly it doesn't sound like that'd be needed, but still)

That's my take. But I'd also really like to hear some opinions on this one.