Wm_Adam_Smythe says the game is fundamentally different if the war god actualy exists as a theistic beign, with its own independent awareness. Will and destiny. In that scenario the war God takes what it wants or needs from the practictioner, permission or not, mortal ignorance or not, contract or not, protection or not. Nothing stands between the practitioner and the entity's vampirism except the practitiioner's decision not to engage at all.
My reply was aimed at Etu's reply, but I have some stuff I could add.
Theistic worship of an entity such as the above described practice appears to take belief as a pre-requisite. If the practitioner's system allows for controls in which the practitioner holds equal belief (or faith in their effectiveness) then control should still be attainable, just in a way that is really hard to prove to someone else.
Both theistic and atheistic examination of Vampyric traits place importance on control of the relationship. I find it interesting that compounding will (believing that you can so that you can which makes you believe you can more so you can more) is the primary modality of control. I find it equally interesting that this "Energy," just may be will itself. The method of control becomes the food source.
It fits nicely with the theme, like a glove really. A Hollywood vampire could be controlled via its food source (like how Sookie controls Bill in TrueBlood) and this archetype is no different.
By maintaining this control....Well the practitioner seems like more of a Vampyre for Sekhem than the practitioner, Holy crap. He has boundless power that builds itself and gives it to lesser beings (Those without physical bodies) in exchange for what Sekhem they have. Mind you this power is limited, and can only be gained through human beings. I'm thinking The Master in the strain controlling Palmer with his White. He dishes out his boundless power in drops, promising to convert him someday into a Strigoi, stringing him along for most of the show. I know its a little abstract, but its a fun flip to consider.