Pivotal Greenplum Database (GPDB) 4.3.x
It is safe to cancel a Vacuum on a Greenplum system, as long as you cancel it using the pg_cancel_backend and pg_terminate_back command within the DB. As the DB is issuing the cancel, then it will safely rollback the DB to before the Vacuum.
But canceling the vacuum will mean the work done it up until you canceled it will be lost and the next vacuum will have to start at the beginning again.
This is the same for all vacuums (Normal, Full, Freeze).
Cancel the vacuum as you would any other process/session running in the Database. Please see the before KB on how to cancel running sessions. Follow this to cancel the Vacuum.
Note- Avoid using Operating System kill command to kill the Vacuum process as it will result in database crash/Postmaster Reset.