the main issue here is that veigar used to be a late game monster like nasus, where nasus would take your objectives while the rest of your team did something else on the map. veigar would join up with the team and kill the enemy team. that was the difference between the two. nasus 2 shots your turrets/inhibs, veigar kills your carries.
nowadays veigar even when fully farmed and beyond a 40minute game he still doesn't one shot people, there is no godly damage to look forward to anymore. he exists as a utility press E as much as possible champion. there is no purpose to farming for 300+ bonus AP when his scalings have all been crippled.
veigar used to be able to blow his full combo and kill a squishy and widely favor any fight. now if anyone builds a single MR item veigar cant do it, no matter how much hes fed and how big he gets, he can not kill someone with 1 MR/hp item no matter what he builds, how late it is, how much hes farmed, making his damage negligable and his sole purpose becomes place the stun let the rest of the team do the work.
thats what killed veigar.
right now, its not veigar winning the game, or anything in particular that you're doing on him, its simply solid team work and good positioning and or punishing the enemy teams bad positioning. tons of other champions just bring more utility and more damage than he does.