League-of-legends – Why getting the first blood is so important

The only role that can viably buy an early Vampiric Scepter is a jungler. This is because lifesteal procs off auto attacks, and in early game you should not be auto attacking creeps because it will push your lane too much.

One of the main ideas behind stacking Doran's is that it provides a cheap advantage for early game harrasment. If two champions with similar base stats are harassing each other, the champ with 2 Doran's will have greater sustain in lane and most likely will win the harrasments. Having more health allows you to win the skirmishes against your opponent, survive jungler ganks, last hit easier, and stay in the lane after a failed jungle gank or harassment.

This does put you behind for getting your first big purchase item, however given that a B. F. Sword (the most common big purchase for AD champs) is 1550g, spending an early 475g on a second Doran's, and consequently getting greater CS (note that after ~25 CS the dorans has paid for itself) and champ kills, can actually allow you to get that B. F. Sword sooner.

It should work, but probably you cannot see it because the numbers are too low to break the threshold for spell vamp visual effect. More details on this forum discussion.