For some reason the 6.5 release not does not state the supported versions..
Does Zerto 6.6 support VMware vSphere 5.5 ??
Please see the operability matrix here: http://s3.amazonaws.com/zertodownload_docs/Latest/Zerto%20Virtual%20Replication%20Operability%20Matrix.pdf to see the supported platforms. Yes, ZVR 6.5 is supported on vSphere 5.5.
Zerto Product Manager