Lucy doesn't really parse these spells right at the moment. otoh, does.
The simple upshot is that these spells change the body-type of a mob, allowing special spells to work on them - Feralize changes the creature to an animal, so animal-only spells work on it. Skin to Vines makes it a plant. They also have secondary effects - Feralize increases blunt damage done to the mob and makes it miss more if it's using a blunt weapon, while Skin to Vines makes it take a bit more damage from fire spells.
(Aside: I don't know if the blunt thing is deliberate. Sony has this bad habit of setting things that affect melee to only affect 1HB when they -meant- to make them work on anything. Several OoW abilities were like this for a -long- time.)