I'll break down that post for you:
"This "rework" ended up just being nerfs to almost everything about him.
w does less damage
e does less damage"
-They left the base damages alone and nerfed the AP ratios a little bit. Not even gonna mention the mana refund on Q.
"passive does less damage until you get a good chunk of AP."
-At level 18, the Passive does like 40 less base damage, but now it has a .4 AP ratio, which does more damage than the old passive if you have more than 100 AP.
"Ult doesn't apply passive and only does good damage when you have a TON of ap, proc your passive on a TANK, then ult him (and only him)."
-They buffed the base damage of it by 25-50, and increased the AP ratio to 1.0 from .6, but now it deals true damage if you set it up properly. Lets see how much benefit dealing true damage is over the single Proc of your passive that the old ult provided. Hell, I'll even throw in 2 procs just for fun.
-So lets say you are against an Ahri, which is a pretty common Vel'koz counter, and you are level 6. She has 30 base Magic resist, and your passive deals 85 True damage per proc (these are old Vel'koz Numbers). So 30 Magic resist is equal to 23% magic damage reduction (we are assuming no MR runes for the sake of this calculation).
-This means that with the same base damage ult of 500(assuming No AP, which is favored towards the lower ratio Koz), Your ult will deal 500*.67=335 damage to her. Add one Proc of true damage 335+85=420. Add one more Proc of true damage( which assumes she already had one mark of the passive on her before, which is the best possible case scenario) 420+85=505.
-So an Ahri that has NO MR RUNES, and sits there for the full duration of your ult will take a whopping 5 more damage from the old Vel'koz Ultimate than the New Vel'koz Ultimate with True Damage. Now lets assume you have at least 15 AP(the amount that Dorans Ring Gives) or that Ahri is running 12 Magic resist Runes, and the New Vel'koz ult deals more damage, given the condition that it deals true damage.
"In addition, now he doesn't scale well with any items, since flat mana is ridiculously less effective on him than mana regen,"
-I agree with this part
"and now he doesn't scale well with magic pen since he does so much true damage instead,
--I agree that most of his damage is transitioning from magic damage to true damage
" and only his ult scales well with AP."
-So having his passive deal true damage on a .4 AP ratio per proc isn't good scaling?
"I was really excited when they said they were going to make some small changes to Vel'Koz, but they completely screwed it up."
-This is an Opinion
"They noted that Q and R were his most interesting abilities, but they didn't make W and E any more interesting (in fact, made them LESS interesting since now they do less damage)"
-Yes, because the abilities that I use for Champion Damage are his W and E. The only reason you used to max W first on Vel'koz was for the easy waveclear. His Q is his main poke spell, and His ult is his all in. The W was always for waveclear and passive Procs, and his E was to get people off of you or keep them still for you combo.
"WHILE ruining his ult which WAS the best feeling when using Vel'Koz (ulting and melting a whole team); you can't do this anymore since his ult does so much less damage." "
-Sure the old Ult did more damage if you caught a lot of people in it, but in my experience, it's rare to catch more than 3 in the beam at any one time. And honestly, you only land one or two of those big ults per game. Its much more common to use your ult on 1 to 2 people (ganking or killing your enemy laner). And I already did the Math concerning a Single Opponent earlier.
My explanations to as why Vel's winrate has dropped is that:
1: Mobility is now a buyable item, and he has 4 skillshots.
2: Itemization is crap for him now: Athenes is worthless on him now, and Liandry's is still really expensive. Health costs have also gone down for other mages, specifically Rod Of Ages.
3: His average playrate has gone up considerably recently, and there are a lot of people playing him since he got reworked, and they don't know how to play him. Vel is a very hard Champion because he has no mobility and several unique skills(Q,W, and E come to mind).
4: Whenever a champ gets changed, it takes people a while to figure out new build paths and new power spikes, and I'd say we are still within this period of figuring things out.
I hope you actually read this before saying I'm full of crap.