Does spell vamp work when used with spells such as Nasus' siphoning strike and Ezreals' Mystic Shot? If so does it add the amount of dmg from just the spell or does it add the bonus dmg too?

Any damage that is ADDED from Q W E or R is vamped. I'm not exactly sure if, say, how Nasus's siphon strike builds up damage with kills would also leech, or if gangplank's attack damage that is added on parrrrlay (or Ezreal's with mystic shot) is leeched, either.

If it does magic damage, it heals you for the damage done, such as lifesteal heals you for physical damage done.

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.

Nasus Gains Juice back from Siphoning strike if he has Lifesteal on.

Spells that do physical damage (Mystic Shot is one of these such spells, if I recall correctly) will not give you vamp.

Spirit Fire will give you a % of life back based on the damage it does each tick.

all right thanks for clearing that up

Im pretty sure that anything that triggers lifesteal is not going to trigger spell vamp. Like Nassus's strike, or EZ's mystic shot.