Hello, I'm actually pretty new to Vel'Koz, but to be fair everyone is, I've had the chance to use him in quite a few matches now and I have to say this is the best way I've seen to build him so far.

As stated you have plenty of choices for your Glyphs but everything else is pretty much set in stone.

Glyph of Scaling Ability Power is my personal favorite for every situation but I don't mind swapping them out for flat Glyph's to just be a bully in lane

Glyph of Magic Resist combined with an early Chalice of Harmony pretty much guarentees that your laning opponent can't burst you, but I don't like these as much because of the build path, it's just too much MR when the jungler can just come in and kill you because he's AD

Glyph of Cooldown Reduction I don't mind these at all, their pretty strong combined with the mana regen from Chalice of Harmony you become so spammy it's just annoying

There are a few options I haven't tried or mentioned, if you're laning against an AD and you have Cooldown Glyphs you can try getting Archangel's Staff you won't have much CDR but your damage should be ridiculous enough to the point where it doesn't matter, if you want to try that out be my guest.

Some people prefer taking Ignite over Barrier but it doesn't make sense to me, I tried Ignite and it never really came in handy, whenever I was bursting someone it was always way out of Ignite range, but Barrier has saved me too many times to count

In short Vel'Koz isn't really a single target burst champion like most AP mids, he's more similar to Kayle where they sit in the back and just do AOE damage during team fights.