Wota is actually a pretty bad, inefficient item for Ryze. Hear me out and don't scream yet...
For 2300 gold, he gets 70 AP which isn't worth as much to Ryze since he is mostly mana-driven. His AP has about 2/3rds the impact as mana does, per gold spent.
So the 70 AP is really worth about 1000 gold to Ryze.
So you are paying 1300g for the 25% spell vamp. How does this work out?
In dominion staying topped off isn't THAT difficult due to fast B, common Bs, and health packs. So the vamp isn't as useful for that as in SR with bigger map.
So the true purpose of WotA is to give you more "effective life" in combat, just like autoattackers use lifesteal.
Does it work?
Kind of. WotA is a bit subpar for many casters because they tend to be at full health when they toss out all their spells, and gain nothing. Then they are on cooldown and don't have much for when the melee guy jumps on them.
Ryze has continuous firepower so WotA actually works for him! Ryze Qs, XIn charges, ryze proceeds to drop his W,Q,E,Q,R,Q,W,Q,E,Q.
In an ideal situation, Ryze can deal 3000 raw damage to the enemy which is 2000 actual which will give him 500 life back. He only gets about 400 really since he can damage them before they start to hurt him.
This is pretty sweet, but this is really all you are getting for your gold remember, plus a modest amount of AP. You are basicaly getting a Giant's Belt.
If you took a Rod, you'd be getting the same AP overall, a bit more health overall, and the 700ish mana would improve your firepower significantly. And Rod isn't really even a great item for Ryze.
The problem with WoTA is that as soon as Ryze pops his Ult, his stuff becomes AOE and only leeches 1/3rd as much. His Ult does give him extra spellvamp though to compensate, sort of.
Also, Ryze has to be dealing damage to get this virtual life! The problem is that he is often CCed which messes up his chain. He also gets bursted to death by 2 attackers. In this case, Ryze often just isn't able to get all that much damage off. Maybe a thousand or so, which is 200 health leeched.
This is why I would not use WotA as a PRIMARY item. I would call it a situational secondary:
Primary items are:
sorc shoes, Glacial Shroud, Odins Veil, Tear.
You will almost always upgrade Shroud in dominion to Heart. In SR you may do other stuff first.
Tear will become AA staff, once you have about 3k mana it becomes cost effective.
So you have 2 slots. Some decent choices include:
Rod of Ages, even taken late mostly for the health booster.
WotA, if their burst and CC isn't too bad.
Abyssal Scepter if they have high magical damage. Abyssal adds a LOT of extra dmg.
Void Staff in theory. In dominion I never ever see multiple players stacking >80 MR. So abyssal is better.
Another AA staff for pure offense, IF you have 4k mana pool.
Warmog for defense (only on SR of course. Pure health with 200 armor and 150MR = badass)
Rylais - Youd get a giants belt for pure health, then upgrade to rylais as a last thing to do.