How to Fix Veigar's Event Horizon
You have three options.
A) Revert the changes (unlikely)
B) Reduce the cast time to 0.25 seconds and the delay to 0.5 seconds. (So he still needs to stop moving to cast it and alerts the enemy to be ready to dodge, but shortens the window to do so)
C) Reduce the size of the entire skill, so that targets caged cannot dodge his W without walking into the stun wall. This way you will gain an either/or benefit situation. They can eat damage or the stun, their pick. Choosing correctly is counterplay.