play aggresive, hit your Q's

first get boots and lost chapter
when you got luden and hextech alternator you're fully set up to poking
with horizon focus you can easily blind poke them

focus on using your ult properly, after activating passive, and for the better when they are CCd
with full build you should be able to Q for ~800dmg with 7s cd, deal ~500 true dmg with passive and ~1500 true dmg with ult

you if you get your combo right you would be able to deal enough damage to win a game changing team fight

vel'koz is very squishy and has no emergency CC to handle ambush
most of his abilities has some delay
Q is slow and hard to hit
W stops casting when dies or get CCd, also even after casting it, it takes 0.75s to deal other half of it dmg, enemies can easily leave its AoE
E takes 0.75s to knockup enemies and it also stops casting while you die, so you need to calculate the right place for it
he can easily die to ganks and in teamfights

vel'koz passive deals true dmg, after procing passive all of your ult dmg is also a true dmg
you don't need to build magic pen when you deal so much true dmg, it just melts most of enemies