Does it work with Rivens abilities? Olaf gets spellvamp from vicious strikes does it proc his True damage / his axe / etc?
Spell Vamp works on all skills because they're all spells. Spell vamp still heals you from skills.
e.g. Olaf's W + Smite = free health. It also reduces the recoil from E because his E heals him with it. His Q still heals too.
Basically, Spell Vamp is life steal for ABILITIES, not magic damage. I.E it won't proc off of Madreds or Wits End, but it WILL proc off of your physical abilities (and smite, which is huge for Olaf).