In general, when a unit dies, all buffs/debuffs that were on them are removed. For Vader's leadership, he doesn't prevent DoTs from being dispelled, he just immediately applies a number of DoTs equal to the number that were dispelled.
I look at that, and it makes sense. L3 died, while she was dead, the game erased her DoTs (so Vader never saw them), then a new L3 appears on the battlefield due to VanChew's unique.
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