Slight tangent, but now I'm curious what it takes to hit Vecna on an 8. Let's see, AC 49 or Ref 47 seem to be the way to go...
Without party buffs, your rogue demigod with a dagger and a Ref attack like Piercing Strike is looking at Pre-Expertise +37 (Level +15, Stat +9, Enh +6, CA & Nimble +3, Prof +4) or +40 after... a less hit-optimized rogue will be around +34/+37, or needing a 10/7 or 13/10, bumping required rolls all by 2 for AC-required attacks.
A dragonborn warlock would more likely be looking at something like +29 (Level +15, Stat +8, Enh +6), needing an 18 to hit Reflex, 20 to hit AC or Fort, and 20 to hit Will with no crits possible. Post-Exp and Arcane Power, the extra +6 would drop those to 12-14 needed.
That's... a truly interesting disparity between the possible types. Of course, you are going to bring some bonuses and penalties into it. Vecna is good about slipping out of some setups and dropping conditions, but probably not quite good enough, especially for the first couple rounds. The long at-will slog later on would be less pleasant.