Since Nunu's abilities have high AP ratio, spell vamp is super effective. Let's check how spell vamp is counted. The formula for single target spells is:
damage dealt * spell vamp = health restored
Imagine Nunu with two Hextech Revolvers which gives 40% spell vamp and 80 AP. His Q at lvl 5 deals 900 true damage + 80 AP. So basically it is:
900 * 40% = 900 * 0.40 = 360 heal
Nunu's Q gives him 345 + 80 health also which makes total number of 785. And it's still Nunu ONLY with 2 Hextech Revolvers. With Deathcap heal is insane but unfortunately Nunu needs Banshee Veil, boots and tons of wards before items like Rabadon's Deathcap and even Hextech's. And Nunu doesnt need kills in early game since he is almost useless in late game. He should give most of kills to a carry and concentrate on buying wards and countering opponets' jungling.
On other hand, Nunu has ultimate which is glorious for farming creeps. You can kill even few minion's waves in a second and run away. Dont be shy, use your ultimate on creeps whenever you can. It has short cooldown and in teamfight it will be interrupted anyway. That technique allows you to buy that additional Hextechs.
Next time we will talk about spellvamp's impact on Absolute Zero. It deals 625 / 875 / 1125! And AP ratio is one of highest in the game: 2.5.