The article referenced below offers insights and answers to your questions regarding Vitamin D and fish oil supplements. The short answer is that while the absolute verdict still needs to be determined, there are potential benefits to taking these supplements, especially if your diet is deficient.
People who were having 2 or more servings of fish per week did not appear to benefit from fish oil supplements. Those who were not eating adequate levels of fish per week(less than 1.5 servings), but took a fish oil supplement saw a 40% decline in heart attack versus the placebo group. Adding extra vitamin D to a diet and lifestyle that already had or produced enough vitamin D did not offer protection against cardiovascular disease and cancer.
So the winner, up to this point is to eat a well rounded and balanced diet. If you find that you are deficient in fish oil or vitamin D, talk to your health care provider.
[Fish Oil and Vitamin D: Your Questions Answered: Shots - Health News : NPR](https://www.npr.org/sections/health-shots/2018/11/15/668223654/should-you-keep-taking-those-fish-oil-and-vitamin-d-pills)