VReplication will only look for tablet pairings within the same cell. If you want to have cross-cell streams then you will need to create a CellAlias that contains the list of potential cells and specify that using the -cell flag in your VReplication workflow commands.
The server side default which determines the candiate types made available for potential selection in a stream is set using the vttablet’s -vreplication_tablet_type flag. The target tablet will use this when finding the viable source tablet candidates.
You can override this on the client side using your workflow command’s -tablet_types flag.
In Vitess 12.0 and later you can specify an order of preference for the tablet types using the in_order: prefix in both the server and client flags. For example, -tablet_types "in_order:REPLICA,PRIMARY" would cause a replica source tablet to be used whenever possible and a primary would only be used as a fallback in the event that there are no viable replicas available at the time.
The default value for some VReplication workflow command’s -tablet_types flag is “PRIMARY,REPLICA” (e.g. VDiff). When a primary source tablet is used it can have a significant impact on your production workloads.