Vel'Koz is strong tbh
I've been playing him quite a lot recently, and he's honestly very strong.
You pretty much run him like Xerath except instead of Rabadons you get a Liandrys. Rabadons is nice as a last item sort of thing in some situations, but in general Vel'Koz has awful scaling and you simply do more damage with other items.
Example of a core build:

And again, similar to Xerath, you should probably bring
![]()
You just have to learn his combos tbh. Being able to W>E>W can often halve someone if you have enough stuff. Just watch that one LCS clip and you'll know what I mean.