Unfortunately the answers in the link provided by Canary do not seem to describe the term as used in the Magistrates’ Court in England and Wales.
When a trial is vacated it means that it will no longer take place on the date set aside for it in the court calendar. This usually happens when one or both of the parties is not ready to proceed and there is agreement between them both that an adjournment (that is, a postponement) would be in order.
If only one side wanted the adjournment and the other did not agree the trial date would still remain, the witnesses still required to attend and the Magistrates would decide whether to allow an adjournment or not.
The fact that the trial has been vacated (rather than discontinued, which would happen if the defendant had indicated his intention to change his plea) indicates that the first scenario I described is the most likely to apply here.
One last thing. In Magistrates’ courts witnesses are not normally summonsed to appear in the first instance. They are asked to attend. If they fail to do so and the Magistrates agree that it is necessary for them to be there they may issue a witness summons effectively forcing them to appear.