As this answer outlines, the Raven Queen was a mortal until she killed Nerull and took his stuff. Why is her portfolio different from his, then? She's got Winter and Fate (Nerull didn't), but doesn't have Nerull's signature Death schtick; instead she safeguards the sanctity and finality of death. What gives?
Nerull was a jerk who wanted to be king of the gods. The other deities were happy when the Raven Queen croaked him—happy enough to raise her to godhood in his place—but didn't want a repeat performance. So they tweaked her portfolio a little, and she later added a couple extra domains of her own.
To prevent her from becoming a tyrant in the same mold as Nerull, the deities elevated this mortal shade to their own ranks and appointed her the goddess of death—but not the dead. Although sovereignty over the dead has been denied her, over the long ages of her reign she has added winter and fate to her domain. [Manual of the Planes 111]
To the best of my knowledge no single god now has the curation of souls as their responsibility. The Raven Queen makes sure souls move on, but there are at least a half-dozen different gods and demons who oversee various afterlives which souls might find themselves in depending on their beliefs and behaviour in life. (Vecna's Undeath portfolio is about keeping souls out of the afterlife and defying Death, so that's not relevant.)