I have just tested the manual integration, but unfortunately it is neither working to switch on vacation mode in UI card nor via service call. As I didn’t find an issue for that, I assume, that the problem is sitting in front of my keyboard.
BUT: It is in UI config. it is in UI, it is in services-list. alarm_control_panel.alarm_arm_vacation is there as the other ones. It is not a custom one from me.
My understanding is that the option of vacation mode was made available for HA so that alarm addons could use it but I believe the function / ability is not available in the standard manual alarm.
That would explain the service function being available but not actions if you only have the manual alarm.