According to the wiki entry for Kanchelsis, deity for vampirism, there is an alternate creation myth that attributes the myth to an article entitled "Core Beliefs: Pelor" by Sean K. Reynolds, appearing in Dragon magazine #346.
It's not much, but I've found the text for "Punishment of the Undead" which describes the myth and redemption (Note: For citation purposes, this is transcribed directly from the Dragon magazine article):
Punishment of the Undead: This fable tells of how some of the first mortals came to turn from Pelor's light, researched evil magic, and became vampires - creatures of the night that can only take life, not give it.
By cursing Pelor's name and turning from him, they made it so they could never look upon him again without being destroyed. The fable points out that this destruction is a result of the undead turning their hearts and souls from Pelor, not because of his wrath.
His forgiveness is total, and the loss of these children caused him great sadness, not anger (although he grew angry at the evil they wrought).
The story tells that Pelor would use his power to redeem them if they asked. This story explains why undead suffer from the sun's light, but if they would turn away from evil, Pelor would welcome them back into his illuminated graces.