Do you mean visitor ID to uniquely identify the unique visitors? It will be available under identity map but if you won't set it explicitly to any field in your schema you won't find it in the payload(it will be available in your dataset though) you can refer to this article if you would like to fetch ECID.
If you're referring to visit Id to identify unique sessions it won't be available under payload as I am aware there is no unique session ID at client side to identify visits once the hit is being processed you will get the visit number in conjunction to each unique visitors in your reports.
you can use the getVisitNum plugin to get the value and store it in a field in your schema.
