[quote name='DarkSageRK']Will I be in trouble if I put my Vit at 8 (for Anti-Force - Passive - -50% damage from Force) and my Str at 8 (for Blitzkrieg - Auto Skill - 25% Mag/Atk increase) for the rest of the game? Or should I bump each up to 11 for Null Fire/Ice/Force? I'm also curious as to how long battles will get later on. I mean, if Magic Yin (Auto-Skill - 1.5x Mag damage, but 2x MP cost) absolutely outshines Blitzkrieg, I might as well just put Str at 6 for Anti-Fire only and pump up Mag/Agi instead.[/QUOTE]
8 Vit is somewhat risky on your first playthrough, plus you'll miss out on the various rise + jumps to boost your attacks, though end-game demons mostly have null/reflect/drain for phys/fire/ice/elec/force/mystic so its up to you whether you want a higher Vit.
8 Str should be fine if your going full Mag versus full Str, though once the Mag resistance demons appear you'll have to rely on your phys party members (which could be somewhat a hassle).
Magic Yin isn't unlocked during the main playthrough, it's unlocked once you crack it from a certain end path.
Hope that helps!