What could be harmless reasons form having ana positive test (1:80). is it definite that it would mean autoimmune disease? could it be because of vitamin d deficiency?

Positive ANA test at lower titer, 1:80 or below, can be seen in 15-35 % of normal individuals. Can be positive in non-autoimmune conditions, e.g., mononucleosis, endocarditis, hepatitis, or neoplasm. ANA positive individuals often get tested for individual autoantibodies.

An ANA at 1:80, at your age is not necessarily pathological. All laboratory results need to be interpreted in the clinical context and the doctor who ordered the tests is usually in the best position to do that. Talk to the doctor who ordered the tests. Vitamin D deficiency does not cause positive ANA.