I'm only Silver II, but I have been a Vel'Koz main since his release. I also just played against a Vel'Koz that had NO IDEA how to play him so I'm really in the mood for giving tips on him. First off, his W is specifically designed to hit champions that are standing in the middle of a minion wave. Secondly, the key to Vel'Koz's early game is his passive. I always take Q at lvl 1 to allow me to poke right from the start. The only problem with this is that it is completely countered by hiding in the minion wave. Hence, at lvl 2 you should take his W, now you can poke them inside or outside of the minion wave and because of the geometry of his Q you can also poke them behind the wave. At lvl 3 you grab your E so that you now have your "full" combo. Now you should start looking for the right time and place to catch someone with your E and if you are confident that you will hit them with it, you should simultaneously cast your W in the same place as your E. This will allow you to get your passive to proc, unless they only get caught by the side of your E, in which case you may only get as few as one point of your passive on them. Now, until you get to lvl 6, you probably aren't going to kill them unless they are just horrible at dodging your spells. But Vel'Koz has one of the highest lvl 6 kill potentials in the game. I know this because I constantly underestimate my lvl 6 damage with every other champ I play and get burned by it. But again, the key to his early game damage lies entirely in his passive. Also, when catching a champ without any minion cover, the proper way to burst is E->W->Q->R. When they are inside a minion wave, you can leave out the Q if you want, but you might be able to catch them with it if your W takes out the minions between you and the enemy.
Now, on how to build Vel'Koz, your first back you should always look to buy chalice. Now that you have this mana regen, you can spam your spells a lot more than your enemy probably can. I also never buy boots on Vel'Koz, he is already immobile, and buying boots isn't really going to help you that much most of the time. However, I have built boots on him at times when I feel myself consistently getting caught by the edges of enemy spells. Note, you should always, always get sorc boots if you do get boots on him. Next, I like to rush Luden's Echo. With your chalice allowing you to spam constantly, you will benefit greatly from Luden's and it will make your burst deceitfully deadly to even some mildly tanky champs. My next item is usually situational, but my core revolves around Liandry's, Rylai's, Void Staff, Rabadon's, Zhonya's, and Athene's.
? I find him to be.... weak and a little clunky.
I think that maybe he should have a slight buff to his AP ratio so that his early game is just a little stronger.
Call of the Void (Q) with the way it is used. Instead of my having to walk around minions to use my W, I would instead be able to lay it out just the way Malzahars Q is.
