Weird Vampiric Touch debuff delay
 

I have an addon that checks debuffs on the current target (amongst other things it does) and it's fine on every class there is, but today I am implementing the Shadow Priest and noticed that it does not detect the Vampiric Touch quickly enough until a while after it has landed. This is a target debuff. A similar issue exists with the Warlock's Doomguard and UnitExists("pet") - I get nils even though the pet is already out. With Vampiric Touch it is weird as it does not happen with say Shadow Word: Pain, or any other class and their target DOTs. Only with Vampiric Touch. When I cast Vampiric Touch I swear I can see the debuff on the target before it has quite finished casting - so it appears that this is not an issue. But the auras say otherwise. So, just like the Warlock, I will use a flag to override the aura which thinks there is no Vampiric Touch when it clearly exists. I would like to know however what its problem is. Has anyone hit on a similar issue? If yes, what target debuff was it?