Vitamin D deficiency is prevalent in adults throughout the world. Epidemiological studies have shown significant associations between vitamin D deficiency and an increased risk of various neuropsychiatric and neurodegenerative disorders, such as schizophrenia, depression, Alzheimer's disease and cognitive impairment. However, studies based on observational epidemiology cannot address questions of causality; they cannot determine if vitamin D deficiency is a causal factor leading to the adverse health outcome. The main aim of this study was to determine if AVD deficiency would exacerbate the effects of a secondary exposure, in this case social stress, in BALB/c mice and in the more resilient C57BL/6 mice. Ten-week old male BALB/c and C57BL/6 mice were fed a control or vitamin D deficient diet for 10 weeks, and the mice were further separated into one of two groups for social treatment, either Separated (SEP) or Social Defeat (DEF). SEP mice were placed two per cage with a perforated Plexiglas divider, whereas the DEF mice underwent 10days of social defeat prior to behavioural testing. We found that AVD-deficient mice were more vulnerable to the effects of social stress using a social avoidance test, and this was dependent on strain. These results support the hypothesis that vitamin D deficiency may exacerbate behavioural outcomes in mice vulnerable to stress, a finding that can help guide future studies. Importantly, these discoveries support the epidemiological link between vitamin D deficiency and neuropsychiatric and neurodegenerative disorders; and has provided clues that can guide future studies related to unravelling the mechanisms of action linking adult vitamin D deficiency and adverse brain related outcomes.