Can low levels of vitamin d cause brain fog? i have this brain fog issue about 10 years now?

Yes it can

Maybe you have an answer so supplement vitamin D-3 and aim for level of 50-70 ng/ml by next 2-3 months. If this fails, further testing is indicated

Low levels of Vitamin D have been associated with negative symptoms of depression, anxiety and psychoses. You can work with your doctor to get to an appropriate dose and therapeutic level for your total Vitamin D level which may be above 30 ng/ml. Monitoring blood tests are needed to ensure that your D3 or total vitamin D level doesn't drop too low or too high.