nonet So sorry to hear about your diagnosis.
You are right, Vit D is thought to have effects associated with both the onset, also the progression of Multiple Sclerosis. Much research has been done and is ongoing. That does not mean that lack of vitamin d definately is the primary cause, or that it definately will help in managing the disease. It may be one of the factors. However to my mind, since you may have a deficiency, and it is fairly straightforward to put that right and it may help with your other symptoms , why not do something about it. Nothing to loose but a definate possibilty of benefit for you.
You presumably must be under the care of your own medical professional, talk to him about your intention to up your vitamin d level. As you probably see on here, I am a believer in testing your level, then taking whatever action you see fit , with respect to raising your blood level, then checking that it has worked, with another test, about 3 months later.
Your doctor may organise the tests for you, if not, you can get it done privately, but in an NHS lab. I have posted many times on here vitamindtest.org It does cost £28 per test.
Oily fish, i.e. wild salmon will have some vit d, farmed salmon only about one fourth of the wild salmon level. You would struggle to get a significant amount of vitamin d from your diet, possibly a little in meat, depending upon how the animal was kept/supplemented etc. I have mentioned dark green leafy veg several times. They do not contain vitamin d, but a very important co-factor, magnesium , in the chlorophyll molecule.In addition to all the other nutritional benefits, folate, vitamin K etc. Greens are very important to human health.
The incidence of MS rises the further you go from the equator, where UVB is strongest and vitamin d levels are higher. A country like Gabon( on the equator ) for instance has,essentially, no MS. It may be that other photo products in the skin may, I stress may , be helpful or protective. So , a bit of sunshine on your skin could possibly be very beneficial, do not burn, ever, but a gradual build up to a few minutes regularly, when possible, on a large area of skin, without sunscreen may be helpful to you as well.
As you will see in my earlier posts, ancestral blood levels were around 120 -140 , and many vit d researchers recommend 100 - 150 as the level to aim for. You will likely need several thousand units daily to achieve these levels, although some people need less, some more. That is why I believe testing is a sensible thing. Rather than just guessing.
I hope this is helpful for you, sorry again that you have that diagnosis. Here is a website which gathers together evidence, research etc concerning vitamin d and its effects on MS <a class="break-all" href="http://www.vitamindwiki.com/Overview+MS+and+vitamin+D" rel="nofollow" target="_blank">www.vitamindwiki.com/Overview+MS+and+vitamin+D</a> .
Again, please consult your own medical professional before taking any interventions on your own.
BTBH