Hi, I'm trying to increase the power level of my deck to play against my friend's vannifar deck and others well tuned semi competitive deck.
The first link is my deck and the second are cards that I think I should add but have no idea how, any suggestions are welcome.
Current Niv Deck
https://www.mtggoldfish.com/deck/1601392
Potential adds
https://www.mtggoldfish.com/deck/1603535
The central idea is to combo Niv Mizzet off with one of the 3 combo pieces curiosity/tandem lookout/Ophidian eye. One of which I forgot to add and is now struggling to figure what to swap out.
The potential list includes cards that I have hence I wonder whether they should go in the deck and how.

I'm going to assume you realize that you may be starting an arms race, and that you're OK with that.

Looking at your potential includes list, the ones I think you should actually add are:
Ophidian Eye (duh)
Drift of Phantasms - can tutor Ophidian Eye and more importantly, Laboratory Maniac.
Pithing Needle - hoses Vannifar
Sol Ring - because strong card is strong.
Torpor Orb - because Vannifar likes creatures that get value when they ETB, and because it says no to Intruder Alarm.
Stranglehold - to hose Vannifar

That's 6 cards. I'd lose;
Coldsteel Heart
Corrupted Grafstone
Fire Diamond
Sky Diamond
Star Compass - these all ETB tapped, and you have a TON of mana rocks.
Fellwar Stone - might not even make a color you can use, depending who you're playing, and doesn't have any compensating upside, like making more than 1 mana, or drawing a card later.
I would also consider cutting a few of your weaker cards, perhaps Opt, and adding a few additional lands. You're a little light.