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I suspect your vignette is from "throttluing" the vlight cone too early after the reducer - thats a large adaptor ring, stepping from IIRC 56mm to 42mm. you should be wide at the reducer exit as per the Tak diagrammes.
Heck, with the FSQ, I don't step down from 92mm exit to M42 until the camera face, and that's with a 70mm extension (all 92mm wide until the interface with the camera - overkill, but no vignette either)
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