Question: What is the name of the geographical region that early Islamic usage often regarded everywhere east of western Persia and where Vistaham helped Khosrow regain his throne after the rebellion of Bahram Chobin?  
Sentence (from "Vistahm"): Vistahm helped Khosrow regain his throne after the rebellion of Bahram Chobin, but later led a revolt himself, which encompassed the entire Iranian East before being suppressed.
Relevant evidence: Yes