If it was a plot by Vecna, no one would know about it to begin with - he is the God of Secrets, after all, and it'd be against his interests to have another Neutral Evil god to compete against for worshipers. So he surely knows the truth, but isn't telling anyone - and if it is a slander, it's more likely to be perpetuated by a rival Good god.
Hmmm... I don't think it would have been a plot spread by another good-aligned god. After all, lying for one's own benefit at the expense of someone else could be considered an evil act, and a lie as big as this (assuming it is a lie) would be a major, major one. I just can't imagine Heironeous, Kord, Ehlonna, or any of the other good gods perpetrating such a scheme. On the other hand, perhaps a misguided cleric of another good god spreads the rumor in order to boost his own deity's number of worshipers.
On reflection, I agree with you that Vecna wouldn't simply reveal so powerful a secret. He's be more likely to blackmail Pelor and force him to give up some of his divine power under the threat of revealing it to his clergy. Oh, Vecna, you devious boney blackmailer.
Just a personal annoyance here.
It drives me crazy when players insist on writing up overly convoluted back stories for their characters and hold up everyone else from playing because it's just soooo important that they get it finished before we begin.
You know the kind of back story I'm talking about. The one where it's like a soap opera and everyone is tied into everyone in some way, shape, or form. Where the player's personal history involves multiple interwoven relationships, schools, has lived all over the world and something different has happened to them to profoundly impact their lives at every location.
The back story becomes this land mine where no other player in the game dare says anything or do anything with this person because it will in someway trigger them to go on a 1 hour back story revealing spiel... That player expects the game master to bring in elements of that characters back story into every play session - because it's just soooo important.
I like to create thorough back stories for my characters, but more for the DM's reference than my own glorification. I agree that having a character thrust his past into play at every opportunity can be super annoying. I think the only exception would be especially proud elves or dwarves, who might be prone to lecturing the younger races in a "When I was young, I..." kind of way. As a DM, having some background information on a character can be extremely useful plot-wise, as one can occasionally "hit them close to home" by having some menace threaten their hometown or loved ones. Dark revelations about a player's bloodline is always fun too.