I don't have Vel'Koz but I love playing with him and I kinda know what his abilities do so lets get started.
This will be updated when I have him

Organic Deconstruction-Passive
INNATE: Vel'Koz's abilities Deconstruct enemies every time they are hit, stacking up to 3 times. Vel'Koz's basic attacks refresh Deconstruction stacks' duration on-hit.The third stack consumes them all, dealing 33 - 169 (based on level) (+ 50% AP) bonus true damage.


Plasma Fission-Q
FIRST ACTIVE: Vel'Koz fires an energy bolt in a line, stopping at the first enemy hit, dealing magic damage and slowing by 70% for a few seconds, decaying over the duration. After a 0.25 seconds delay, and while the bolt is in the air, Vel'Koz can reactivate Plasma Fission.
SECOND ACTIVE: Vel'Koz splits the energy bolt in two, each firing in opposite directions perpendicular to the original bolt's trajectory as well as copying its effects.


Void Rift-W
PASSIVE: Vel'Koz stores a charge of Void Rift periodically, up to a maximum of 2 stored at once.
ACTIVE: Vel'Koz opens a rift to the void that cuts through the ground in a line, dealing magic damage and leaving behind a trail. After a 0.25 seconds delay, the entire trail detonates, dealing magic damage to enemies standing upon it.


Tectonic Disruption-E
ACTIVE: Vel'Koz hurls a disruptive anomaly at the target location, dealing magic damage and suspending enemies hit for 0.75 seconds. Enemies close to Vel'Koz at the time of impact are also slightly Airborne icon knocked back.


Life Form Disintegration Ray-R
PASSIVE:Deconstructing( Organic Deconstruction) enemies marks them as Researched for 7 seconds. Basic attacks and ability damage will refresh this duration.
ACTIVE: Vel'Koz channels for 2.5 seconds and projects an energy beam in a line. The ray's trajectory updates itself over the duration to face the cursor. Moving or reactivating Lifeform Disintegration Ray immediately ends its effects.
The ray deals magic damage in 0.25 seconds intervals to enemies caught in its wake, and Slow icon slows them by 20% for 1 second. Organic Deconstruction is applied every 0.7 seconds to enemies hit, up to 3 stacks. Researched Researched enemies take true damage instead.

Needed you have a lack of moblity so you need to get out somehow

Adds some damage over time and denies some heal from like Heal or Astral Infusionand Aria of Perseverance

it can help you keep enemies in place its genneraly good and its damage reduction helps with survivablity
I take Flash and Exhaust but I think ignte is ok too but Flash is mandatory
or if you get Ignite and Exhaust you need Hextech Rocketbelt

AP runes preferably with some CDR otherwise I DON'T CARE.:P

Pros
-Small knock up
-Good kiteing potential
-TRUE DAMAGE
-Really fun if you like the long range poke and burst

Cons
-No true escape
-Needs some mana
-Very squishy
-Has long cooldowns

I hope this guide helped you to play Vel'Koz in the support role please feel free to use the build in other roles but jungle and If so replace eye of the oasis with Morellonomicon
I wish you GL HF :-)