The damage that is inflicted to your opponent’s Life Points as a result of battle with “Gravekeeper’s Vassal” attack is treated as effect damage instead, not Battle Damage. So your opponent cannot use “Kuriboh” or “Waboku” to prevent the damage, for example.
A monster destroyed by “Gravekeeper’s Vassal” in battle is still treated as “destroyed as a result of battle” (for “Mystic Tomato”, “Nimble Momonga”, etc.).
If you equip “Fairy Meteor Crush” to “Gravekeeper’s Vassal”, and attack a Defense Position monster and inflict Battle Damage to the opponent’s Life Points, the damage is treated as effect damage due to “Gravekeeper’s Vassal”’s effect.
You cannot activate “Barrel Behind the Door” against damage inflicted by “Gravekeeper’s Vassal” because its effect is a Continuous Effect and you cannot activate “Barrel Behind the Door” against Continuous Effects.
Netrep Rulings:
Damage inflicted by “Gravekeeper’s Vassal” will trigger the effect of “Dark Room of Nightmare.
“Rod of the Mind’s Eye” will not alter the Damage done by “Gravekeeper’s Vassal.”