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Upon further investigation, the vassal's enchantments also do not show up in the Ledger of all current enchantments. This also indicates that there is no mana upkeep associated with them.
One of my units previously already had the 'Tireless March' enchantment on him. Upon obtaining a new vassal with 'Tireless March' it was retained, and now I have two different units with it. It is of my personal opinion that no enchantments should retained upon vassalization as a matter of balance, though I can understand just limiting it to unique conflicting enchantments.