Because these abilities are Auto-Attack enhancements, not spells, technically. If an ability reads, "(insert champion name)'s next basic attack will an additional X damage", then the ability counts as a basic attack. However, if the ability reads, "(insert champion name)'s next basic attack will an additional X damage and does another effect", the ability is an on-hit effect ability, thus applying Spell Vamp.
For example, let's compare Nasus's Q and Leona's Q.
Siphoning Strike: Nasus's next attack within the next 10 seconds will deal bonus physical damage.
Shield of Daybreak: Leona's next basic attack deals additional magic damage and stuns the target for 1.25 seconds.
Nasus's Q is an auto-attack enhancement, an on-hit effect. That being said, this ability will apply Life Steal. Leona's Q is an auto-attack enhancement as well, but her's is an on-hit effect ability. It does something a basic attack cannot. The physical damage from Leona's basic attack will apply Life Steal while the bonus magic damage will apply Spell Vamp.
I hope that answered your question, or at least got you to understand things a bit better. Sometimes it takes a little research to tell what your ability is going to do. All these champions are so different! Best of luck to you, fellow Summoner.