Holy Jesus, Veigar's Ult damage is ridiculous.
Walks up to enemy ADC at 75% HP Presses R A massive number appears on the screen: "1879" "You have slain an enemy!"
You don't even need the enemy to be at low HP; that's just for tanks with 200 MR so they actually take execute damage. This thing is a point-and-click ability that deals 75% AP INSTANTLY. It deals 100% AP if the enemy is below 78% HP. Now, I don't really want to nerf it, b/c Vei is supposed to just walk up and nuke enemies.
So what I think is that the base damage needs to go down significantly, and the scaling go up. Example: 50/100/150 (+32% AP) damage, increased up to 250/500/750 (+160% AP) damage against targets below 33% HP. Just make it so he can't instantly 1-shot enemies regardless of hp; it's supposed to be an execute move, not a 1-shot move. So with this Vei can properly use his E-Q-W-R combo.