I don't really think that they can be seen as the same since, on a comparison of what happens with each, they actually are opposite to each other. A zombie has to have a period of zero oxygen to the brain; a brain death. Although it is never said as far as I know, it displays the action of what happens; no cellular regeneration, and disruption of involuntary responses. For instance, the brain of a zombie has erratic response to pain, muscle movement, hunger, sensory signals, and adrenaline rushes. They don't say, "Naw, I don't feel like eating this guy, you go ahead.", or move as if rigor never set in, or say, "OUCH!! That hurt you big bully!!", so the involuntary responses have been altered from a brain death. Since they do not improve, there is no cellular regeneration; staying as they are is the best that a zombie can hope to be. The possessions still hold on to the key factors that retain life, a zombie does not. With a deadite, the look of decay is more along the lines of a reaction; a fiddleback spider bites and the result is decay on areas around the bite. Remove the demon, or remove the venom in the bite, the reaction goes to. Granted some still maintain rot areas from a spider bite, but they are more sensitive to the venom. It would be closer to place a deadite in the class of vampire since they do keep the functions.Even then, the vampire is acting singular, while the possession is completely acting with the force of a second party. That was how I always looked at it, but could be wrong.