The First Evil's plans never really included a resurrected Slayer and a souled vampire. The First Evil knew that good and evil would become even more balanced (the presence of thousand of Slayers worldwide) due to a resurrected Slayer and a souled vampire - which is why it went after Angel in S3. After Angel left, it figured that balance would not be reinforced . . . until Buffy was resurrected for a second time in S6 and Spike became souled.
The First decided to make its own plans to upset that balance. By killing off the Potentials and starting an apocalypse, its goal was to manipulate and lead Buffy to that scythe, so it could wield it through her. Or get rid of Spike as an ally of Buffy's one way or the other. That is why it tried to manipulate Spike into siring Buffy in "Sleeper", if Buffy didn't kill him for killing off the town's numerous citizens. It even tried to use Robin Wood into getting rid of Spike.
Thanks to Buffy, the Slayer line, along with the balance of good and evil was redefined (the idea of one Slayer, amidst thousands or millions of vamps, demons, etc. did seem ludicrous). Thanks to Spike's soul, the Sunnydale Hellmouth was closed and the First's plans to change the world's dimensions ruined. Of course, Angel's attack on the Circle of the Black Thorn probably gave the First hope, since the attack spurred W&H to make that last and probably catastrophic attack on L.A.