Nothing about Zereth Mortis was really about the aesthetics of death in the strictest sense - it was about the afterlife, in this case, the various afterlife realms that actually constitute the Shadowlands proper (and not just the four Covenant realms we primarily dealt with). Zereth Mortis was about fabricating the realms themselves and populating them with appropriate flora and fauna, which we saw in their protoform roles prior to being placed within a given realm. So if, for example, an orc needed a "happy hunting ground" type of afterlife realm, Zereth Mortis would fabricate it and stock it full of animals to hunt and so forth. That's what those giant glowing bronze/gold orbs are all over Zereth Mortis - protoform realms in the process of being created, which the giant machinery at the heart of the zone then "launches" into the In-Between to create a new afterlife where the Arbiter can send souls.
Zereth Vitae would likely have a somewhat similar function, but would instead fabricate flora and fauna to actually fill the physical universe and its various planets as opposed to afterlife realms within the Shadowlands.