Vel Koz isn't a high skill cap champion. His skillshots aren't even that hard to land, seeing as he has a CC combo that allows him to easily land combos.
If you land one, you can land the rest of the abilities.
A very high skill cap champion would be someone like LeBlanc. Landing a combo on Leblanc is harder now without the silence and to escape.
I'd have to put Vel Koz to a medium skill cap champion. A hit or a miss. If you land it, easy follow ups. If you don't, then you miss the combo.
While on a champion such as leblanc (No more silence) landing a full combo in succession may be difficult, unless you catch the enemy by surprise.
Ahri is another AP champion with a higher skill cap, while she is mobile, her CC is harder to hit than Vel Koz.
The 0 mobility on Velkoz is not really a worry, since if anyone attempts to gank him he can easily either E the ground infront of their path/ or Q infront of the path. Or he can E himself if for example theyhave a gap close (Pantheon stun).
Other champions such as Ahri (Pre 6) have to be careful of when they use their charm, because if it's down (long CD) she has no escape.
The only disadvantage I see Vel Koz is; is having the laning phase if he is being camped.
While I agree we are in a mobile meta, vel koz is actually anti-mobile. His slow is really effective, his E disengage is good, and his ult is at a high enough range that he can hit most champions who try to run away.
The combo you have at bot lane is very odd, and most likely succeeds due to the adc/support not being able to keep at a distance; or they do not know how to kite a volibear.
A flip into a vel koz E into a Q could spell instant kill if you time it nicely.
Most adc's would be out of range for that, and most supports would CC volibear or try to stop him. (Popular supports: Thresh/blitz/Leona/sona/soraka) all of these have some sort of range or some sort of cc to stop volibear or to counter the volibears engage.
