Vel'Koz Runes explained
There's one set of runes for Vel'Koz on patch
11.15
that is the clear favorite for LoL players and pros alike. The best Vel'Koz Runes on the position 
Mid are Sorcery as the primary path and
Inspiration as the secondary path with a pick rate of 88%. Besides runes you should take a look on Vel'Koz Build and if you are interested in pro matches take a look at Vel'Koz Probuilds. If you want to discuss
Vel'Koz then you should visit the champion subreddit.
Primary path on Vel'Koz
Sorcery empoweres abilities of Vel'Koz and increases the available champion resources. Most of high elo players – and our LoL pros in the the major regions – have agreed that
Arcane Comet is the best keystone rune to run on
Vel'Koz. This keystone rune adds more damage to your abilities and increases your power in the laning phase. This is usually followed up by taking
Manaflow Band,
Transcendence and
Scorch.
This is usually followed up by taking
Manaflow Band for additional mana and mana regenaration.
Transcendence for some more Ability Haste.
Scorch increases your control in the early game.
Secondary path on Vel'Koz
When it comes to your secondary path, most
Vel'Koz players prefer to go with
Inspiration.
Inspiration opens creative opportunities on Vel'Koz troughout the match. On the
Inspiration path, the best runes for Velkoz are
Minion Dematerializer and
Cosmic Insight.
Minion Dematerializer will help you to control the position of the minion wave.
Cosmic Insight grants summoner spell and item haste.