Now this is just conjecture based on my own interpretations, but based on the JK story the ritual to consume the life in the galaxy required 2 things. A certain amount of deaths over a certain time and a huge amount of death in a short time, probably creating a disturbance in the force which Vitiate can use to feed or something.

On Ziost his spirit controls beings and then uses to kill other beings then posses more, it's a cycle until I think it reaches a point where he's able to consume the planet.

You know, this might actually be a good explanation for it. Hopefully someone will clear it up because from the Revan novel and every other source it always seemed to involve a force related ritual, no Zildrog at all, but I suppose it's possible that information could be retconned as Nyriss could've been wrong or not fully aware of what had happened.

Force ritual and the use of Zildrog aren't necessarily excluding one another. Even if Vitiate used a particular mean to end people's lives, I think he would still need to perform a ritual in order to catch the spirits of the deads. In the end, no matter said murdering means, that's not what will be remembered. People who are Force Sensitive can feel the death of a whole world through the Force... if someone siphon the energy of the population of an entire planet, someone is bound to feel it. The latter is much more troubling than the former, since there are a lot of way to effectively kill entire populations in the matter of seconds, while there isn't that much appearence of one being eating said population.