Two criteria to determine whether to used 缺乏 or 缺少:
In the sense of syntax, 缺乏 can only refer to abstract ideas or groups of item (somewhat abstract, I think) and it cannot be used to refer to numbers of items or any specific item. 缺少 can refer to anything which 缺乏 can refer to, plus it can be number specific. For example:
- 你缺乏合适的经验。 >>> abstract idea
- 我缺乏信心。 (I lack confidence.) >>> abstract idea
- 我们缺乏物资。 (We lack supplies.) >>> group of item
- 我缺乏维生素 C。 (I lack vitamin c.) >>> group of item (one does not have to have "a specific molecule" of vitamin c, thus 缺乏 can be used here.)
All of the above 缺乏s are interchangeable with 缺少.
- 这个城市缺少一个泳池。 >>> number specific
- 我缺少几本书。 (I lack a few books.) >>> specific items
However, in daily speech, 缺少 is often shortened to 缺, it sounds more natural.
In the sense of meaning, 缺乏 means "lack" in a more serious or more severe sense. 缺少 (or just 缺) means "lack" in general. For example:
- 你缺乏勇气。 >>> emphasise the seriousness of "lack". In this case, using the shortened form (你缺勇气) might seem a bit weird since this sentence probably is used in a more serious context.
- 这个城市缺少一个泳池。 >>> just "lack" in general, not in an urgent or serious way. Using 缺 (这个城市缺一个泳池) is also fine.
- 我们缺乏物资。 >>> probably want to emphasise the "lack" when one is in a bad scenario.
One only uses 缺乏 or 缺少 when one wants to emphasise the difference in meaning. Otherwise, people just say 缺 when refer to the general idea of "lack". In the written language, the shortened form 缺 is used less often since it is less formal.
In short, 缺少 = 缺, and 缺乏 is a specific and emphasised version of 缺少. It is a bit like 缺 means "lack", and adding 乏 makes it more serious (and can only refer to more abstract ideas :p).
Hope this helps.