I suppose that:
vehicleController.lstRemindDaysBefore
is a List of some Day Objects, if I'm right than the code should look more or less like this:
<p:selectOneMenu id="firstRemindDaysBefore"
value="#{vehicleController.vehicle.DaysBefore}"
valueChangeListener="#{vehicleController.reminderBeforeDays}">
<f:selectItem itemLabel="Select One" itemValue="#{null}" />
<f:selectItems value="#{vehicleController.lstRemindDaysBefore}" var="day"
itemLabel="#{day.label} Days" itemValue="#{day}" />
</p:selectOneMenu>
or if it is a List of Integers then:
<f:selectItems value="#{vehicleController.lstRemindDaysBefore}" var="day"
itemLabel="#{day} Days" itemValue="#{day}"/>