Currently, all items that provide Spell Vamp also provide AP, which is wasted on champions that would love to have Spell Vamp but can't make use of AP, at least not very effectively. Granted, there aren't very many such champions, especially if we limit it to champions without AP ratios at all. But they do exist -- Riven, Talon, and Zed come to mind. Even some champions with AP ratios think Gunblade is just a total waste because of how little AD it provides by comparison to other AD items -- Aatrox, Pantheon, Graves, Renekton. Soul Knife and Soul Chakram seek to correct that.
Soul Knife
Long Sword + 280 Gold (680 total)
+12 AD
Unique: +8% Spell Vamp
Soul Chakram
Soul Knife + B.F. Sword + 600 Gold (2830 Total)
+60 AD
Unique: 30% Spell Vamp
Unique: Dealing spell damage increases your AD by 5 for 6 seconds. This stacks up to 5 times.
Hard to derive gold efficiency when Long Sword and B.F. Sword are both the item used to derive price of AD, and they derive different numbers...
Soul Knife
Long Sword + 280 Gold (680 total)
+12 AD
Unique: +8% Spell Vamp
Soul Chakram
Soul Knife + B.F. Sword + 600 Gold (2830 Total)
+60 AD
Unique: 30% Spell Vamp
Unique: Dealing spell damage increases your AD by 5 for 6 seconds. This stacks up to 5 times.
Hard to derive gold efficiency when Long Sword and B.F. Sword are both the item used to derive price of AD, and they derive different numbers...