A good Veigar player will Q farm extremely well early game, and become a beast late game he continues to Q farm or get kills on enemy champs, even if he doesn't kill them with Q as his Q's passive gives him AP per kill from his other moves if they kill the champ.
I don't agree that his Q needs an extra passive that gives him 1 AP per assist, that will make him extremely broken as he can just participate in team fights and get AP from any assists he obtains.
Here's a scenario you need to understand if he gained 1 AP per assist
-Veigar has 100 CS in about 12 minutes, about 75 of those minions were Q farmed, a mixture of wraiths and cannon minions in involved. Assuming about 15 or so that is from the larger creeps, his AP, just from the Q farm is 105. Adding runes and masteries, say he just has a Doran's Ring, his AP is roughly 180.
-He has a kill to his count, boosting his AP by 2 since he killed someone with R not Q. However, he has 5 assists due to jungle helping him counter gank and roaming bot to help them out. Now he has 5 more AP. Assuming Veigar hasn't backed yet, his AP is now 187 at level 8.
-Veigar has the following items at end game: Sorc Shoes, DFG, DC, Zhonya's, Athene's, and Void Staff. His Q farm gives him +250 AP as he killed the enemy ADC a couple times with Q. He now has close to, if not above, 1k AP just from Q farm and his items. Adding to this, he is 9/4/17. Meaning that his AP is incredibly high, thus making him a high priority target (hence Zhonya's).
End result: Veigar is now impossible to deal with, even with MR, and unless he is focus fired fast, he will face roll the enemy ADC, or burst down the enemy mage in no time flat. Keep in mind, a good Veigar knows how to use Event Horizon to their advantage, and that makes W easier to land, which WILL sting since it scales with 100% of his AP (1k landing on your head is never a fun thing).
What I feel his Q needs is similar to Nasus's Q, an indicator to how much he's Q farmed, which will clue other players on whether or not he needs to be stopped or not. However, more players know if Veigar is left alone to Q farm, he will become a major threat end game.
While it is a sound idea, but that will render him broken, and become a high priority pick/ban at higher ranks. He's a Q farm champ, if he is to gain an AP boost from assists, then his AP scaling must be reduced, and LOSE the ability to gain AP from killing players with his other spells as they give 2 AP per kill he gets.
In my honest and blunt opinion, he's fine currently. His skill set is designed to boost his AP, so as long as he Q farms and gets kills regardless of which spell kills the player, he gets AP, and the Needlessly Large Rod line is essential on him due to their high AP boosts.
tl;dr, 1 AP/assist is a bad idea. Will make him broken.
