add a field to each of the 2 subtables to hold the value of how many days they have taken off. You can then use an after update event on the end date and have it use the datediff function. the syntax would be similiar to this, substitute your field names where appropriate.
datediff (d,me.startdate,me.enddate) the d specifies for it to return days, the date diff function has several different types of data it can return.
This is getting confusing, i'll try to write out all the code, you will need to substitute your field names:
dim totaldays as string
totaldays = datediff( d, me.startdate, me.enddate)
me.newfield.value = totaldays
you can then use an update query to update the total available hours for each employee by subtracting your new field value from their total available. Hope this helps