It kinda is. The issue with Spellvamp is that all vamprism effects are more or less useless on burst (Most mages) but incredibly useful on DPS (most marksmen.) Where Spellvamp had problems, however, is that it turned waveclear into full heals, hence the 33% effectiveness on AoE.
Of course, Lifesteal has the exact same issue, functions on more or less on champions with the exact same core goal (strong DPS and medium range), and just offers needless complexity (So these on-hit effects benefit from lifesteal, but these don't? And this abilities do, and these don't? Que?), so when you get right down to it they need to be rolled into the same stat that benefits from ALL damage dealt, but is only 33% effective against minions to limit sustain while rewarding dueling and harassment.