Veigar's mana regeneration is increased by 1.5% for each 1% of his missing mana.
^^^^^^^^^^^^^^ the actual reading
you're misquoting him, it increases his mana regen amount, not restores max percent mana.
so he actually needs mana regeneration for his passive to increase that flat amount of regen to become meaningful.
if he was at 0% mana, he would have an extra 150% of whatever the amount of regeneration he has, say dorans ring + level 1 base of 6mana per 5. so 9mana per 5 turns into 9 + (9x1.5) = (13.5 + 9) = 22.5mana per 5seconds. worthless. thats hardly anything. thats 1 Q every 15seconds, for someone with "extra mana regen" hes hardly gaining any mana back.
so he builds a tear to gain both max mana and mana regen, and then his mana issues suddenly aren't a problem anymore, blue buff? spam away.
his passive just doesn't help him when he needs it, and it should be replaced with something else, because his passive doesn't help with his low base pool and higher mana costs.
he Q's like 5 times or he uses his stun and meteor once each and hes down to a quarter mana at the earlier levels, thats crippling and doesn't let him use any other abilities other than Q if he gets dived on, so now he will basically only ever have enough mana to Q again or hes just last hitting for quite a while not gaining any bonus AP.