Well Vecna doesn't exactly [i]need[/i] a reason to do some interplanar bullshit. He's the God of secrets, just say he's testing something, and the PC's were specifically chosen by Vecna to be test subjects because they all possess some sort of "potential" or "gift" that the secret needs. Like they're immune to the effects of the spell that causes the traveling, and Vecna keeps casting the spell on the party to test his hypothesis. Feel free to go completely crazy. You can do just about anything with magic and a god of secrets.