At lvl 1 you don't have Hex. At lvl 2 I picked up Sleep Hex and now she does Hex and then Ice Ray. Later she is going to be my Ice nuker, going to take Winter Witch levels for more bonuses.
Bought 4th character after 1st voyage was done. Ecclesitheurge with Fire and Good domains (it works as it should in second game), this is going to be buffer and and fire nuker. I noticed permanent 10 HP bonus is not automatically given to characters that join the party afterwards so I guess it is important to fill 6 slots sooner than later.
Question for your masters of builds, how to do I give my finess Vivi dex bonus to damage? Just with Mythic levels or few levels of rogue?
Also in first voyage I found some fire thingy that is obviously some story/quest item but the Helmsman does not want to talk about it. What do I do with it?
I'm not a master of builds so take my opinion with a pinch of salt, but honestly it depends.
You'll probably only get one Mythic Feat and two abilities. If you want to spend the only Mythic Feat on Mythic Finesse you can, there are a lot of precision immune creatures so sneak attacks aren't always useful and guaranteed damage is helpful. Personally, I really like Mythic Two-Weapon fighting, but I can see the argument for taking Mythic Finesse.
As you suggest, you could instead dip into rogue. I'd go 4 levels for debilitating injury if you do, it's really helpful for two-weapon fighters to land iterative attacks. You won't reach 12th level for greater mutagen on this run so you won't miss out on that by taking rogue levels. It really just depends if you need the higher level extracts or not.
Basically, if you can't think of a mythic feat or ability you'd like to take more than Mythic Finesse then pick it up, otherwise consider rogue if you can live without access to 4th level extracts.