here's the damage for if he landed his W and R, which would be his most damaging combo possible in that setup
level 5 W = 300 + 100% AP ; level 3 R = 325 + 75% ap up to 650 + 150% AP
that is up to 950 + 250% AP
even at 1000 AP and base magic resist of around 30, that's 2950 damage before resist or 2269 after, that would be enough to kill you, but he also needs 1000 AP, which is very far into the late game
let's assume he does
70 AP
120AP
90 AP and they might have enough for 
this comes to 364 AP before runes or etc and you would need at least 100 stacks to reach 500 AP
if he somehow has 500 AP, this comes to 2350 (which includes luden's) before any effects such as MR. With 2 very expensive items and 1 full item at the least, he could come close to killing you...if you are a marksman in his range for no reason. If you are at base mr of 30, this entire build would reduce you to 3 magic resistance, or 97% damage taken, but again, you would have to be a marksman in veigar's range for no reason for this to happen
even with all circumstances in veigar's favor, that only comes to 2279
#in addition to that, his ultimate scales by missing hp, so it is not always at 650 + 150%
if we factor in that it was his W that hit, they would be at roughly 52% hp, or a 48% increase on his ult with 1050 remaining 325 + 75% (multiplied by 1.48) = 481 + 1.11 AP = 1036 damage
runes renders you dead, but again, this scenario is ridiculous
#here's why: you have no magic resistance, you are the squishiest class in vegar's mid-range, veigar has 100 stacks and 3 items in a meta where games start to end at around 20 minutes
reduces this damage by a ton,
(which is a free item slot...) reduces it even more so