I had that Vivitiar 2x converter for my Hasselblad. I never used it with the 80mm lens, since I owned the Hasselblad 150mm Sonnar. I did use it many times with the Sommar to make a 300mm lens. There was a significant contrast loss with the 2x converter, and some sharpness loss too. It was acceptable, but a real 300mm lens would have been better. I couldn't afford one, and didn't need the focal length often, so the 2x was ok.
Fullframe shot done with the 150mm Zeiss Sonnar and the Vivitar 2x Converter. Shot on Fuji Acros 100, developed in Rodinal 1+50, and scanned with a Nikon 8000ED scanner with glass carrier.
Below are two 100% crops of the scan, showing detail.