One main difference between vectors and deques is that the latter allows efficient insertion at the front of the structure as well as the back.
Deques also do not guarantee that their elements are contiguous in memory so the at-style operator (indexing) may not be as efficient.
Note that the difference is unlikely to matter in practice for smaller collections but would generally become more important if, for example, the collection size increases or you're modifying it many times per second.
