I know this is going to sound very extremist and bad: I don't give a damn. What I mean by this:
Your proposition, and the author's in a way, is that even though he is a compilation of the fragments of souls, he still lacks the "destiny" to become the Holy Son and thus it's one of his achievements nonetheless. (Death Magic is unrelated to the compilation by the way, I will later in this comment explain what I think happened and why Vandar ended up with that particular skillset).
The rebutal would be the presentation: Even if that was the author's intent, it doesn't show that way at all.
a) He gets the title of Holy Son because he is recognized, sure. But he also gets favors from every Vida supporting demigod just because he is who he is, before he even gets to interact with them. That is a Fated One, not an achievement he got on his own effort.
b) Even though he is not "destined" to help Vida in any way, she still pulls his soul into her cycle, had she not done so, he would have probably not become Holy Son at that young of an age and maybe not at all (because he would be culturally immersed in the Alda religion and it's perfectly possible for him not to meet a Vida believer in his whole life, actually).
c) Everything Vandar does up to the point where he gets the demigods' help (and some of that too) could be done without the premise of him being a compilation of the champions. Even if the author tries to divert the idea that it's because he is a compilation, there is no reason for him to be a compilation other than that, so it's naturally asociated. In short: the reason the author included the fact he is a compilation (because it doesn't affect the story in any other way) is to justify him helping Vida. This is a very important point and I will repeat it a third time with another phrasing: Him being a compilation doesn't help the advancement of the plot AT ALL except to justify him being fated to help Vida. Which is, actually, quite poisonous to the reader and the work: it opens the doors to people like me, who would otherwise not complain about this point at all, to complain. And it pretty much pleases no one, I think, in a way that the same character without being a compilation would not.
About the Death Magic ... Well, it looks to me like a recycled idea of some sort. Mostly because it manifests itself in two contradictory ways which are never explained (because the internal works of any system in the novel are always left half hidden and half broken ... ).
On one side, you have the Abscense of Purpose and Meaning (stupid reference, ignore it please): because he was stripped from everything in a context where he should have something, he developed an affinity with nothingness, or the void, or however you want to call it. This is consistent with how the early Death Magic works on every observable instance except spirit communication and control (which can be also seen as a perversion of this concept, but since it fits better into the other way of manifestation, I prefer to consider it to be part of it). This manifestation of Death Magic controls the abscense of things more than it's presence: a flame that absorbs (causes an abscense of) heat, a barrier that absorbs ( '' ) kinetic energy, he removes (abscense again) the aging from a body, and ultimately he can "break" (cast into abscense) souls. During the fight against the orc it's even hinted that Vandar can't use Death Magic in any offensive way against him, since there is nothing significative enough he can remove. (Even though at this time there are also uses of Death Magic that don't fit into this category, like the familiar skulls).
On another side, you have Death and everything related to it. There is no explanation as to why Vandar would gain power over death. Spirit communication is gained because he is unable to communicate with the living and somehow has affinity with the dead. The only explanation to be given is because of the "Death Attribute", so it's a semantic relationship (because Death Attribute magic doesn't, actually, have anything to do with death in a way that any other attribute doesn't: it can cause death just like any other, and it has applications unrelated to death or the dead like any other). Essentially, he gains control over the dead because his magic is CALLED "Death Magic". This later evolves in him being able to communicate and control spirits, raise undead (who are not exactly undead, but almost), and gives him a completely unnecesary enticement skill which, by the way, he should not have (Vandar doesn't interact with the dead more, or in a more meaningful way, than any other person does with any other thing. And any spiritualist (which do exist) over the course of their lifetime would interact more, and probably in more meaningful ways, than a couple of months old child did with literally 5 spirits).
End of rant.