Everyone needs to ensure that they prevent vitamin deficieny by eating nutritious food.
But apart from this there can be some people who can still be susceptible to deficiencies. The elderly are one of the main group and this can be for many reasons - ill-health that can lead to malabsorption of some vitamins and minerals but also financial hardship that leads to eating a poor diet. In addition, elderly people just often don't have that a great apetite and this can lead to nutrional deficiency.
Pregnant and nursing mums can also be prone to vitamin deficiency as they require higher amounts of vitamins. Some vegetarians can also be prone to lack of vitamins depending on what foods they eat/prefer but this usually also involves lack of minerals in the diet as well such as iron, calcium and so on.