After clear the story i need explain everybody pls i need explain what happen after we left echo omg i cant, and when cameron say that hes a vessel what is that mean omg
HUGE SPOILERS AHEAD
If I understand correctly, the whole thing with Cameron being the "vessel" for Devon is a vision Cam sees of what's happening in the alternative timeline, where Cam failed to survive Brian's gunshot.
The version of the present in which Devon lives on either possessed by something that resembles Cam (I think its not actually Cam, because think about Sam) or just lost his mind (which is less likely, because I don't think that things are ever that simple in Echo VNs, even if it IS a reasonable explanation).
Like, remember how Cam saw a vision of himself getting shot in the head and then grabbed at Brian's gunshot so that it doesn't happen?
Well, what he saw was a version of future that does actually exist, and the one he succeeded to avoid, thus creating a "fork in the road" of time.