They perform evaluations for purposes of civil litigation, as an aspect of economic damages.
Economic experts calculate the value of those earnings over time, so the difference, if any, between the two income streams is clearly understood.
Those who act as vocational/economic experts blend the two disciplines, and offer testimony in both arenas.
Qualifications to testify in court as an expert in the field of vocational rehabilitation are fairly strict and related to State certification and licensure.
[1] Ultimately, the rules of evidence in the jurisdiction presiding over the civil case prevail.