his:
Dark Matter
Active: Veigar calls a great mass of dark matter to fall from the sky to the target location, which lands after a 1.25 second delay, dealing magic damage in a 112.5-radius area.
Magic Damage: 120 / 170 / 220 / 270 / 320 (+ 100% AP)
yours:
Dark Matter Rain: vig'maw shots dark blasts out of his staff you get to choose where they fall it does 60/100/180/240/300 damage plus 50% of your ability power.
Going by what you said there... you get more then one shot... i assume you planed on giving 3-5 shots max... even then... if he gets 2 shots your actual damage potential is 120/200/360/480/600, with no delay while dropping... So you want a more powerful clone of Veigar's ability.
hers:
>Inner Flame
Active: Karma fires a ball of energy that detonates on hitting an enemy, dealing magic damage and slowing surrounding enemies by 25% for 1.5 seconds. If it hits an unseen enemy, the detonation effect will still be visible.
Magic Damage: 80 / 125 / 170 / 215 / 260 (+ 60% AP)
yours:
>Soul burn: Vig'Maw Shoots out a black soul out of his body exploding if it hits a wall or a champion it does 100/200/300/350/400 damage scales off 40% of his ability power if it hits a wall or a champion.
Much more damage from you... WAY too much to be exact, plus yours will blow up if you hit anything at all... so you don't even need to worry about hitting them, just something near them... which is basically karma's ult form of this one.
yours
E---------Dark matter shield: Vig'Maw shields one of his team mates shields them for 8 seconds and shielding 10/50/70/100/190 of there health scales off 60% of your armor.
An AP that builds AR? so only two items build into helping you with both stats, one is worthless on your champ and the other is an item almost any make will build... plus most shields that are used on another person scale with AP. Though the base numbers are... low... plus they scale so weird... 10+40=50+20=70+30=100+90=190... It isn't strong enough and scales too erratically.
yours:
>R---------Healing front: Vig'Maw uses all of his energy to make a healing blast on target team mate he heals them for 300/400/800 health scaling 30% off your Ability power
A MASSIVE, single target healing spell? i understand it... but personally i don't feel it would fit with a void creature. Not saying this is overpowered as Soraka has a larger heal when you factor in #of targets... though you have that strange scaling...
Also, no cool downs, mana costs, range on anything... those all make this champ incomplete, thus it isnt easy to criticize since it could end up broken from those alone.