Louca,
In general, I would not use the decompressor cable unless you are turning the engine with the kick start lever at the same time. Keeping a small amount of pressure on the compression lever on the handlebar while turning the engine over will allow the cam lobe itself to lift the cam follower rather than letting the decompressor cable do all the work. As you turn the engine the decompression lever will easily go to the handlebar and once it is there it is usually not under any severe stress.
The cable should be easy to replace.
View attachment 43957
The end of the cable slides into the end of part number E218, and you should be able to see the hole in the hex end of that item.
I thought there was a sleeve and a cable anchor over the end of the E218, but they are not listed here. I will leave the details to someone who has changed one recently.
The Comet is a wonderful Vincent to own, but it was not well-represented in the Vincent literature compared to the twins. It is often difficult to find drawings related to the Comet.
Best of luck,
David