Not entirely certain the 4e Op board is where I put this, but it IS a 4e trick...
To start, you play a swordmage. It doesn't matter what swordmage. At level nine, take the daily power Be Gone. Only one other thing in the build matters, if you want certain success: take a fighter multiclass feat.
Do whatever you want for the rest of Heroic and Paragon. If you feel like it, try to get as much accuracy with your attacks. That'll let you do the trick sooner. You could try now, and hope for a natural 20, but I do believe that deities are immune to the powers and attacks of all non-epic characters.
Now, at epic, go Eternal Defender, entirely for the level 30 ability. Either now, gambling on your attack roll, or at 30, fight Vecna. For this step, you need a friend and a bag of holding.
Have the friend stand within 5 squares of you, holding the bag open, with a readied action to close it after you go. Now, attack Vecna with Be Gone! If you hit, which you do at level 30 thanks to Eternal Defender, teleport Vecna into the bag. The friend closes the bag. He is now trapped.
What can you do with a deity in a bag? Well, other than having defeated a god with a level nine daily, you can use the Aura Cannon! Opening the bag will let the aura out--but only in the direction where there's a connection to the material plane. Now, Vecna has a teleport speed, but teleporting requires line of sight, yes? So open the bag behind a shield. He's trapped, and you can blast your foes with his aura!
I chose Vecna because his medium size allows him to fit into a standard bag of holding. You might be able to talk the DM into more expensive bigger bags. These would allow you to collect the entire pantheon, one per day.