Those ideas aren't terrible, and would tweak the 'Veigar doesn't get enough AP from Q' and the 'Veigar's skillset suggests you shouldn't get killing blow with R, but rather with Q'
But it wouldn't fix him on the whole, because there are still a few more issues regarding his role.
His Ultimate has two effects aside from straight nuke, one that deals damage equal to 25% of the target's mana, and one that replenishes his mana if he gets a kill with his Ultimate.
- The addition of damage equal to mana doesn't help a lot in the current method of the game. Most casters will sit around with 900 Mana mid-game and not stack mana items. That only gives 225 damage extra against a caster, which, sure, it's a big help, but it's not going to win him any fights.
- The first +Mana item most casters buy is Rod of Ages. Any additional damage his Ultimate would do is entirely negated by the bonus health given by this item.
- The mana drain portion is not a good benefit for him. Most players will go for Mejai's Soulstealer rather early. This item already has a mana drain mechanic to it, making the additional effect to his ultimate negligent anywhere other than at the tail end of the early game.
So if anything needs changing, I think the Ultimate really needs a change.