In VDBSMVBasic or Advanced, set the State ID under Vore State. Whenever the enemy/actor casts a skill that applies that State, it should work when A) the target's sprite is no longer visible and B) the target cannot be target normally. The Pred State is optional, mainly used if you want a skill to "remove" the vore relationship from the pred instead of the prey.
Anyway, you would then place the <tag: x> in the Actor/Enemy noteboxes, either if they are the caster (Pred) or the target (Prey). For example, Enemy Slime and Enemy Toad both cast the same Vore Skill but can be defined with different turn lengths in their noteboxes.