To be honnest a lot of veigar plays GLP rather than luden. Yet luden need to be charged.
So let's still take an ideal scenario where veigar use luden : luden deals 100 + 10% AP
Veigar ult damages deals 1,5% more damage per 1% hp missing for the target.
https://i.imgur.com/58mA78C.png
The breakpoint is at 1650 AP
I Otp veigar. I never reached such a high level of ap in my life in a fair ranked game (not talking about when I climb with a smurf and make the won game last on purpose to stack ap).
I reach full build one game out of 5, but I rarely get full damages, I often get a single defensive item. (That would be stupid to be fed just to get deleted by the ennemy Vi or nocturne without any chance to even cast a spell).
When i'm over fed and full stuff i'll be at 1200-1400 AP (happens one game out of 20) maybe)
Never have I ever seen a vayne not buying at least an MR item when facing a fed veigar.
So we can theorycraft all day long, but at the end of the day, veigar never deletes anyone with R only anymore.