I can bet a few things despite not having played this champion.
Riot likes to reuse character playstyles, especially true if the said design wasn't well received/didn't work mechanically the first time. The easiest example would be the sheer number of attempts of "melee carry" they've done.
All that being said, its safe to say Vel is the latest spin on the "artillery caster" type character. Like previous incarnations such as Xerath, Lux, Ziggs(arguable), he is characterized by having the following.
- Long range
- Skillshot dependant
- Horrible mobility
If you ask me, his weaknesses (poor mobility, avoidable attacks) are even more exemplified than his predecessors.
- Vel is entirely based on skillshots, every one of his moves is avoidable.
- His only defences are 2 CC moves that are not reliable/difficult to aim.
- His ult roots him in place for a fairly long period of time, worsening his lack of mobility.
If you want a TLDR version: Whatever counters other artillery casters does the same to Vel'Koz. Infact it arguably counters him even harder.