If Cheney gets tarnished by the Plame investigation, it might be expedient to drop him.
The only way I see VP Cheney stepping down is for health reasons -- if the doctor was concerned about his heart condition then I think he should step aside and think about his health. There is no way that Pres Bush would just drop him from the ticket IMHO.
What evidence do you have that Cheney would be associated with Plame in any way, aside from her husband's scurrilous lies?