"Verdolmetschen" isn't old-fashioned: Duden says it's "umgangssprachlich, auch Fachjargon" (colloquial, also specialist terminology - "Jargon" doesn't necessarily have the pejorative sense it does in English). I'm surprised that it's described as "colloquial", but it's certainly a standard word in interpreting - specifically, it tends to be used in transitive phrases (i.e. with a direct object), whereas "dolmetschen" is usually used intransitively (I've only very rarely seen dolmetschen used transitively).

e.g. "Ich musste bei der Konferenz dolmetschen" - I had to act as interpreter at the conference

but "Ich habe die Reden der Konferezteilnehmer verdolmetscht" - I simultaneously translated/interpreted the conference attendees' speeches"

This description of an interpreter's work here uses "verdolmetschen" several times.

The etymology via Turkish is very interesting!