I don't have many Vista panels in the field, but I got a cll the other day other day from a non-client who aid they were having a problem with what I am guess is a Vista panel based on some of what he said.
He said when he presses the away button the system arms and switches to the stay mode.
It ain't odd its a great feature, you can change in programming. If you have Auto Stay enabled if the client arms the system without opening a delay zone (theoretically leaving the home) the system will arm STAY instead of AWAY.
The panel may indeed be a Honeywell (Ademco) Vista. Napco does the same thing (programmable option). If you don't violate a delay door during the exit delay period the panel switches to "home" mode with interior zones disabled. It helps prevent a lot of user error false alarms. It is a required feature, called "Unvacated Premises" under the CP01 Control Panel Standard - Features for False Alarm Reduction. I cite it below:
184.108.40.206 Unvacated Premises An option shall be provided which allows the security system to Arm in the Stay Arm mode if the Exit Time expires and no exit has been made. The default setting for this option shall be that it is enabled.
Note: When although it is a required feature, you are not forced to implement it. CP01 allows the installer to disable the feature though the manufacturer is required to provide it and the default setting must implement it.