From Dark and Darker Wiki
The healing potion example shows that Vampirism applies only at the end, however the Healing Over Time formula state's that Vampirism is equal to the Magical Healing Mod variable.
The healing potion example completely ignores this portion of the formula [(1 + Physical/Magical Healing Mod)] and does not explain why Vampirism is not included here.
Either the Healing Over Time formula should not include [(1 + Physical/Magical Healing Mod)] or the healing potion example is incorrect.