The Niche role of WoTA

Will of the Ancients has always been a fairly difficult item to balance, and not 'just' because Vladmir has healing amplification synergy with the item. Spell Vamp as a stat is incredibly difficult to balance because in order to use this stat effectively your champion must meet four different criteria that when combined mean Vlad and 'maybe' 1-3 other champions use the item above and beyond what other champions ever could and thus making the item feel good on other champions makes it excessively overpowered on these.

1. Must be AP.
2. Must be Sustained Damage instead of Burst.
3. Must deal this Sustained Damage through single-target Spell Vampable abilities, or if AoE damage, must be enough raw damage and hit enough targets to make it function 'like' single target. Sorry Lich Bane.
4. Must be on a champion that survives long enough to apply this value (i.e.: already has some innate tank and/or evasion (ex: troll pool)).

Vlad additionally has 5. "Actual synergy with the item.", but there's a long list of criteria that make Vlad work better with a WoTA than they would even 'without' that being factored in.