There are a couple of ways you can handle this.
One way would be to define the motion detectors that you want to be active while you are at home as a perimeter zone (zone type 03). That will treat them the same as a window contact and trigger the alarm instantly, even in stay mode.
But the Vista actually has two stay modes: Stay and Night Stay. In normal Stay mode, all the interior zones, like the motion detectors, are bypassed. But in Night Stay mode, it will activate certain interior zones, such as the motion detector on the stairs, but leave the others bypassed.
For the zones that you want to be active in Night Stay mode, you add them to Zone List 5. You build the zone list using Menu Mode *81.
You arm the system for Night Stay mode by entering the security code, followed by pressing the Stay key (key 3) twice. Or, you can set a function key to arm Night Stay mode. You could also set a function key for Step Arming that will sequence through Stay, Night Stay and Away modes for each time you press the function key.