One is usually picked over the other based on the specific type of application being written, how much data it needs to store, and how quickly it needs to be able to retrieve it. For the sake of speaking the same vocabulary, let's look at both these storage mechanisms in brief.
A vector is a contiguous block of memory that contains successive pieces of data at regular intervals. Easily the most ubiquitous example of a vector is the string variable (char* or char), which contains a sequence of characters (bytes) one after the next.
char foo = "bar";
Here, foo contains the letter 'b'; immediately afterward, you would find the letter 'a' in foo and so on ending up with a NULL character '