Tell me if I've got the basic formulas wrong

TestEndDate = (VactionEndDate - VacationBeginDate) + TestStartDate

Secondary to that, you'll want to use DateDiff() and DateAdd() to
actually perform the calculatons as they're designed specifically to
work with dates. If you need the TestEndDate to fall on a weekday, you
can use DatePart() to test if the date falls on a Sat or Sun and if so
add 1 or 2 (using DateAdd()) to the inital TestEndDate to shift it.

Below are two functions.  CalcWorkDays returns the number of working days
between two dates.  AddWorkDays returns a date that is a number of working
days from a date.  Both exclude Saturdays and Sundays.  Both include
reference to a table named Holidays which has a date field named holdate.  
Both functions also exclude any dates found in that table that are within the
range of dates being examined.

Function CalcWorkDays(dtmStart As Date, dtmEnd As Date) As Integer
'D Hargis
'Calculates the number of working days between two dates
'dtmStart - the first day to include in the range
'dtmEnd - the last day to include in the range
'Returns the number of working days between the two dates
'Both dates are counted if they are working days

Dim intTotalDays As Integer ' Counter for number of days
Dim dtmToday As Date        ' To increment the date to compare

   intTotalDays = DateDiff("d", dtmStart, dtmEnd) + 1  'Start with total days
                                                       'Add one to include
First Day
   dtmToday = dtmStart                                 'Initiate compare date
   Do Until dtmToday > dtmEnd
       If Weekday(dtmToday, vbMonday) > 5 Then         'It is Saturday or
Sunday
           intTotalDays = intTotalDays - 1             'Take one day away
for Weekend day
       ElseIf Not IsNull(DLookup("[Holdate]", "Holidays", _
               "[Holdate] = #" & dtmToday & "#")) Then 'It is a holiday
           intTotalDays = intTotalDays - 1             'Take one day away
for the Holiday
       End If
   dtmToday = DateAdd("d", 1, dtmToday)                'Add a day for next
compare
   Loop    'Until dtmToday > dtmEnd                     All days have been
compared
   CalcWorkDays = intTotalDays                         'Return the value
End Function

************************************
Public Function AddWorkDays(OriginalDate As Date, DaysToAdd As Integer) As
Date
'D Hargis
'OriginalDate = First Day to calculate number of working days from
'DaysToAdd = Number of Working Days to add to OriginalDate
'Returns the date that is the last working day for the number of days
'To look back, pass a negative number of days
'If 0 is entered, the current date is returned

Dim intDayCount As Integer
Dim intNotADay As Integer
Dim dtmReturnDate As Date
Dim intAdd As Integer
'Determine whether to add or subtract
   Select Case DaysToAdd
       Case Is >= 1
           intAdd = 1
       Case Is = 0
           AddWorkDays = OriginalDate
           Exit Function
       Case Else
           intAdd = -1
   End Select
       
   intDayCount = 0
   Do While True
       If Weekday(OriginalDate, vbMonday) <= 5 Then    'It is a weekday
           If IsNull(DLookup("[HolDate]", "Holidays", _
               "[HolDate] = #" & OriginalDate & "#")) Then
               intDayCount = intDayCount + intAdd
               dtmReturnDate = OriginalDate
           End If
       End If
       If intDayCount = DaysToAdd Then
           Exit Do
       End If
       OriginalDate = DateAdd("d", intAdd, OriginalDate)
   Loop
   AddWorkDays = DateAdd("d", intAdd, dtmReturnDate)
End Function