im not sayin ur completely wrong but he's not that bad... he has huge cc and when u said or typed that vel koz has a huge slow i hope u ment the the annoyin debuff slow and his w just need to land the slow or e to hit the second part of the w(he even has 2 charges of his w!!!) :) his w can stop channels, and i can do a decent amount of dmg,thx 2 his passive, without his ult.
sure he has low mana cost but thats coz vel haz this spaming potiential(which i luv) so really those low mana cost add up 2 wut... 200 idk coz i dont watch my mana xD... his q is pretty slow i guess but only so u can reactivate and let it split early to annoyin them with the slow xD.
sure his ult is really annoyin when ur enemys cancell it -_- but thats just how vel koz works. i luv how everything abot vel koz is pretty much useful.... even his auto attks r useful it resets the timer on the passive and the best part his auto attck and ult goes thro wind wallz xD (hy5!!!) o ya vel supp i guess it works...but he's not really a supp coz he just takes all the killz from his ult xD ik i can cancell it but its ks right(kill secured) xD
but i dont like how they made vel SO slow i mean sure ya thats his weakness but really... i mean thats a straight up advantage for zed ahri kassadin yasuo(nvm yasuo isnt that bad for me :P) akali brand(is sort of same as vel coz of the burst and the squishy but his w and stun is annoying) fizz kat leblanc and etc.
for me vel koz is just a champ thats fun to me ( i main him so i sea many unique things in him) atleast they r not gonna nerf him...right?! ps im getting battlecast vel koz and maybe creator viktor skin ♥

ps 2.0 talon is my worst nightmare he silences(and also tps to u) slows u turn invis then yeh... u have no option but to hug tower which is borin... and just shoot Qs :) and for yasuo it doesnt look like vel's W shouldn't be blocked coz it looks like it comes from the ground and not a shooting arrow right? o well..
the point is vel is fine sure buff him if u want but he's fine...
ps 3.0 i dont like his death animation sure its... different, whichi like but just starin at vel koz without his eye is creepy :P