Are vector clocks useful in centralized systems?
No, there's no need for a vector clock in a centralized system.

A vector clock uses a $N$-vector of timestamps, where $N$ is the number of computers in the distributed system and the $i$th component of the vector block is a timestamp chosen by the $i$th computer.  In a centralized system you'd use a $N$-vector with $N=1$, so it just reduces to a single number instead of a vector.  In other words, instead of a vector, all you need is a single timestamp, where the timestamp is chosen by the central computer.