The point isn't that vel'koz Q is easy to land, it's that it's easy to land from 1050+ range, allowing his enemies no opportunity to retaliate to it. They're a lot like karma Qs in that way, except from even father away but not aoe (though it can hit 3 targets).
Veigar can only do the same with his stun up, which has a relatively long cooldown and makes him so weak while it's down that he has no pressure until it's back up, and he still has to wobble up to 650 range to land his Q against them while they're stunned.
Again, they serve different purposes. The only thing they have in common is that they both have strong disengage, but unlike veigar vel'koz can disengage with all his abilities, not just his long cooldown stun. Veigar is a one-trick-pony. He has his stun+everything combo, and that's it. Even that isn't very scary unless veigar is farmed, and it's not exactly suited to harrassment. Vel'koz is more about the constant, sustained damage+slowing from 1000+ range, combined with strong all-aoe medium range abilities. Veigar left alone in a teamfight might stun several people and even kill a squishy by himself, but vel'koz left alone in a teamfight might kill half your team except the tanks, and even the tanks should be low before the fight started thanks to vel'koz constant Q spam on their face combined with liandry's.
I'm not saying ve'koz is this god-support. Even if I did want an AP support I'd never pick him over morgana (except for fun; played so much morgana lately, she's getting boring), and probably not over karma, though I might consider picking him over zyra. I would pick everybody else listed so far over veigar, though, except - again - for fun.