There are several spells that apply damage over time effects to your opponents, such as Cassiopeia's poisons, Teemo's Toxic Shot and his bombs, and so on. Do these effects activate your spell vamp, since they are begun and technically the effect of an ability?
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Yes all damage over time effects trigger spell vamp immediately, for the entire duration and percentage of damage dealt:
Damage over time abilities (DoT) apply full spell vamp.
Source: http://leagueoflegends.wikia.com/wiki/Spell_vamp
For single-target DoT spells, spell vamp will be applied to the total amount of damage dealt by all ticks.
Source: http://leagueoflegends.wikia.com/wiki/Damage_over_time
The only skills that don't benefit fully from spell vamp are area of effect (AoE), which apply only 1/3 of the spell vamp effect.
