These aren't necessarily mutually exclusive. I have full rewards from this event already and barely used any AP, and my net seed usage has been break-even.
But you're right, but my suggestions weren't a priority list. They were long-term strategic goals that aren't listed specifically in my guide right now, and therefore are something he might not have come across yet. But certainly--outside of events--Vine is one of the better things to be spending excess AP on, especially since getting to that point in the story will unlock a whole bunch of jewels. And that extra -10% DEF will make Advent Ultimate bosses a lot easier with an incomplete weapon grid, which takes time to build.
For team, I'd go with:
Mordred + Sniper Shot
Gaia
Cu Chulainn
Oberon
Zephyrus
That'll get you to -50/-35 ATK/DEF, and -50/-45 with Vine. I favor Mordred since debuffs missing can be fatal, and Blind is really nice when it sticks. Andromeda or Joan are fine if debuffs are sticking reliably.