What are version vectors?
Version vectors are a way of each node in a cluster communicating its local version number to all other nodes in a cluster. They are also known as vector clocks.
In essence, when a node A passes a message to another node B, A includes in that message what it knows about the version number of all nodes in the cluster. Because it contains values for all other nodes it is an array, or vector, of version numbers. Node B uses this information to update what it knows about the version numbers across the cluster. It can use this information to work out a global ordering of events from across the whole cluster.
In turn node B includes this updated information in messages it sends to other nodes, including to node A.
Search for vector clock. There are a great many explanations, both academic and practical.