So I've learned that a Damage Bonus: Maximum actually means Damage Bonus vs Everything: Maximum. When I say this it may not mean anything to you, but think about in combinations with armor and weapons. Some of this may be obvious to you, and this is more to the ones who didn't know about this, and there are many as I've talked to a few.
I had 1 piece of Shadowsun (gives Damage Bonus:Low) and my Callahan has a UV vs Beast VH, when I hover my mouse over "weapon slot 1" at the bottom of my equipment window, and it shows "Damage Bonus vs Beast: Ultra", which means that the Damage Bonus from the Shadowsun applies on top of the UV to add up damage.
What this means is that if someone were to have a Full Skolver set (Very High Damage Bonus) along with the Barbarous Thorn Shield (Medium Damage Bonus) to make a Maximum Damage bonus, having a Cold Iron Vanquisher would gain no damage boost, as it will already be at Maximum.
I haven't tested the actual damage, so if someone with that setup along with a Leviathan Blade test the damage on Undead, in theory the Levi would be doing more damage even to the Undead at this point. If the CIV still does more damage to Undead, then that could mean one of many things. There have been threads like this before, but no one has actually tested this.
1. Maximum isn't the damage cap
2. The equipment window is wrong
3. Damage bonus is an entirely different increase than "Damage Bonus vs Everything"
Your right i think, Assuming levi does more normal dmg (does it?) than the cold iron, than if a damage bonus maxes out, then against undead a levi would have an equal bonus (I mean if they were all maxed out) to the cold iron, and since the cold iron does less normal damage, levi would get more toltal damage multiplied. I know for a fact that damage bonuses multiply, so if they can get maxed, you're correct. I don't think the "maximum!" means that you can't add any more though, I think it means that that's the highest bonus any single peice of equipment can have. So If I had a set each with "damage bonus: Maximum!" then my sword would get double maximum damage bonus. This being said, I've never seen if this could happen, or if it would say something like "maximum x2!" or just show 2 maximums. If it isn't that way, then it should be made that way. Prehaps make a new fourm in the suggestions?