A tour that has been called to confirm and wishes to reschedule but has no firm date is marked with an 'R' on their tour booking (survey/manifest) record. This will not count as a no-show. It will not be a cancellation.
You could mark it as a cancellation, then create a new tour booking record for the person when they book again.
Don't forget to cancel the minivac legs out if it is a mini-vac, a pure cancellation, not a reschedule. Then add in the open dated booking legs so that you know it is still an active mini-vac.