As a Vel fan, the changes were a strategic buff, but overall a nerf. His skillshots are much weaker (except for his q) by themselves, but if he procs his passive the burst is higher later on in the game and he is able to use an ult with a 125% AP true damage beam. Before he was struggling as a burst mage with low enough CDs to be DPS and enough waveclear to be a siege/anti-siege mage while never really surpassing the other mages in any one of these aspects. His base damages were phenomenal and his supporting abilities actually weren't bad either.
Post-rework Vel is now supposed to be anti-tank, but excels at bursting even more now than he did before, leaving him as a tank-shredding burst mage whose CDs are still low-enough to almost make him a DPS mage. With unreliable damage sources, his skill floor has been raised and his skill ceiling is way up there now. Vel's damage is almost disgusting if he can hit his abilities on someone reliably, so a good Vel can easily take down 50+% of your HP level 2 or 3 with his early combos, and his scalings keep that true for squishies at most points of the game. With the "research and disintegration" theme that they put on his passive to allow his ult to deal true damage rarely being necessary against squishies, and usually being needed vs tanks, which he can melt now way better if he hits his skills.
As for his total niche now, I believe he is just a really well-rounded immobile mage. He still has waveclear, scaling, poke, skillshots of all sorts, CC, and burst, and I am of the opinion that he should be much cheaper to purchase and used as an ideal beginner mage to help new players learn how to use those things (also, having Ryze as the 450 IP mage really doesn't make the most sense. He certainly isn't the easiest to pick up or learn from and he's not too forgiving).
Overall, I am satisfied with the new Vel much more than the old. Hitting one skill shot is meh right now, but hitting three feels fantastic (more so than it used to) with the higher scaling on his passive. He does everything you could want in a mage and he does it with more ease than most at the cost of performance quality (a lux will out-burst a Vel, and a Cass should out-DPS Vel, but he will outburst the Cass and out-DPS the Lux, and his cast times are non-existent except for ult). If you want to play him, I reccomend getting a Rylais and Protobest withing your first 3 items (if I'm doing really well i get Ludens first, if not I get those two) and then some CDR and you literally are ok against anyone. Assassins don't even wreck him too hard with the new Protobelt and his e. He's a very well-rounded mage and is currently my favorite of all the mages.