I know of a few forum members from over a decade and a half ago who added Vit D3 into their arsenal of "weapons" to keep their PCa under control and sadly some have passed and some are still alive, in one case 15 years since diagnosis.
Admittedly all of them had treatment of one kind or another but in all cases they made drastic lifestyle and dietary changes in addition to vitamin, mineral and herbal supplements. It may have worked for some and not others but as there is no proof of the efficacy of the changes they made being beneficial, it will have to remain in the realm of being either extremely unlikely or vaguely possible!
I've stuck to my regime of changes and supplements for over a decade and added a few extras into the pot for good measure as time goes by and although I can offer no evidence, I feel the psychological benefits of my daily regime help me in some way. A bit like not stepping on the cracks in the pavement if you want to complete your journey safely.
None of us will know for sure what diet or supplement is best to help control PCa or even help prevent it in the first place, until such time, as previously stated, large scale studies over many years are carried out and I suspect by then the treatment of PCa will have moved on to a point where the condition will no longer be life threatening and the need for changes in lifestyle and dietary changes will be superfluous.