good point with constant damage, but dont forget thats it became a skillshot, so its not very reliably (for fast champion, or champion with jumps or if creeps are there (you cant shoot through 2 minion)
To his passive.
Once you have about 1000 mana, from the roa, he doesn't really need to buy mana regen
Thats... not how the passive works... you must buy
or
, because his passive works on Mp5s not on max mana (prozentual not max)
And more health wont help you as Veigar, if the enemy hits you you are defenseless if you have 3000 HP lategame, it qould still only take a few seconds to kill you, he isnt a tank (and cant be played as one). After his stun is out he is defensless, sincehe has no blink, dash or anything else AND the stun can be dodged, blinked over it (by MORE than 18 champion) or just flashed over it. if you can touch Veigar and his stun is out... then he is as good as dead.
I will give it to you that you said you never played him after his "rework" but you should play him, to be able to contribute to him, or his playstyle.