I find http://www.uesp.net/wiki/Skyrim:Kill_Hern to be a strange quest, in that I would've expected it to be much more suitable to be a Dawnguard quest, rather than a Dark Brotherhood quest. Maybe I could understand it more if it was a vanilla quest, but the quest only becomes available once you have Dawnguard installed. Anyone else think it's more suited to the Dawnguard?

Not really. Anyone wanting to see him dead may not know that he is a vampire, or simply doesn't know of the existence of the Dawnguard and turns to the DB. Maybe he is a vampire too; if so I doubt he would ask the Dawnguard to do the dirty job...

Actually, that quest IS a vanilla quest, i played that quest several times before i got Dawnguard.

Anyway, none of the expansions adds quests to the vanilla factions.

Ah, so it's just their looks it changes?

Yes, DG adds the glow in the eyes.