Actually they have the same meaning, but in linguistics "缺少‘’ usually modify countable things (eg. human - 人，money - 钱，machine - 机器……). And "缺乏" modify abstract or actions (eg.Confidence - 自信， courage - 勇气 ) ps： the countable things i meant is in Chinese not in English.

缺乏比缺少更加的书面一点。

Besides the other explanations, one other subtle difference is that 缺少 just means "a lack of" but 乏 implies that there is a fault in the lacking of.

“缺乏”语义较重，强调极少或没有. “缺少”语义较轻，多指在数量上少一些，有时也指没有。

"缺少“和”缺乏“，都指的是数量不多，甚至没有，但是”缺少“的语意较轻，”缺乏“的语意重，表示极少，另外，”缺少“一般用于具体的事物，”缺乏“可以用于抽象的事物。