So this is how Spell vamp works (as from tests)
Spell vamp will heal you based on how much damage your spell did (magic damage).
So even though Akali's passive adds magic damage to her auto-attack, it does not apply as a spell vamp cause its not a spell.
Spells that deal physical damage do not heal you, such as Pantheon's spear shot, Ez's mystic shot, Pirate's parrrley.
Spells that will heal you do include whatever bonus damage that is delt as spell damage. So i didn't have time to test, but if Nasus's siphoning strike deals magic damage and the bonus he gains also deals magic damage, then spell vamp should heal based on his Base damage + Bonus.
In short, the above post is correct,
Magic damage spells = spell vamp.