Is combination of vitamin d3 & vitamin a in recommended dosage more effective in enhancing immunity as oppose to taking vitamin d3 alone?

Both are needed: Studies show that both Vitamin A and D3 are necessary for immune response. Deficiency in either one along with high levels of the other can cause hypervitaminosis syndrome where the immune system is actually depressed. Recommendations from the US Institute of Medicine suggest an average daily intake of 400–800 IU Vitamin D, and 700-900 mcg/day of Vitamin A. Of course a well balanced diet is vital.