Can low levels of vit d cause fatigue, pain, worsening of slurred speech (pt suffered stroke ~ 10 yrs ago) and general 'spaciness'?

No. What is the reason for the vi d in this case?1 far too many people take supplements self prescribed that at best are useless. Stop the medication and see if it was not prescribed and contact your pcp if it was, but the simple answer is no !
Not an old problem. Stroke progression occurs in the first few days to weeks after the event. 10 years delay means something else is happening. Considerations include stroke extension, drug use, hydrocephalus, seizures, sleep disorders, blood oxygen problems, systemic disease (such as thyroid, anemia, cancer). Low vitamin d levels may affect those symptoms but probably not in a major way.