Interesting suggestions @Ymiraku and I think they are totally fine since Cadence mods only affect the third hit anyways, so the damage conversions would be fine in my opinion.
I do agree with @fordprefect that maybe it should have Bleeding Damage instead of Physical Damage as Vitality/Bleeding go together with sets and items very well. I think doing these changes would be good for Vit/Bleeding DK.
I had thought about Blug’rug’s Plagued Edge this past week and I came up with the idea to have it give some Bleeding Damage mods to Harbinger of Souls since it already gives ranks. That could help reinforce it’s Vit/Bleeding stats and make it work better for Bleeding DK.
Making it a Dagger could help as well so you don’t need to dump points into Cunning. 