Well, even though i dont think Liandrys is optional on Vel'Koz. Maybe it isnt worth to buy it early on but when it comes to mid/lategame i dont think that you can go without it.
Liandrys has to be a core item on Vel'Koz cause its passive synergises great with his abilities and Vel'Koz over all is scaling incredibly well with MPen due to his low AP-scalings.

I dont see Rabadons as a core item.
Rush Athenes, get Void Staff and then go for either Zhonyas or Liandrys or whatever you need. Also im surprised you didnt mention Rylais as an optional item, cause it can work pretty well in certain matchups.

People just aren't stacking enough health early on for it to be cost efficient enough in my opinion. It isn't bad though so if it works for you then there's no reason to change it.

Gragas: I actually think this is a favorable match-up for Kha'Zix 1v1 but he can set up ganks like no other. Be very careful about him playing a little bit more forward than usual as if you get knocked back, chances are is that you're dead.

I like your guide and i agree with you in most choices, except one little thing :
I think Liandry's Torment should be purchased before Rabadon's Deathcap;
Most of Vel'Koz abilities do the CC to proc Liandry's Torments extra damage,
also his AP ratios aren`t too high