These are the highlights, but search for !!!! in both WB projects. I used !!!! as a marker for lines I changed
'!!!
Call Application.Run("'Time Sheet Data.xlsb'!List_Assignments", combb_vertical.Value)
and
'!!!! 1. Added Thisworkbook
'!!!! 2. Added 'dot' to Rows
'!!!! 3. Added parameter to List_Assignments
'!!!!
Sub List_Assignments(Vert As String)
Dim lastrow As Long, i As Long, x As Long
Application.EnableEvents = False
lastrow = Range("L4").End(xlDown).Row
ThisWorkbook.Sheets("Base").Range("D4:D50").Clear
x = 4
With ThisWorkbook.Sheets("Base")
For i = 4 To lastrow
'!!!!
If (.Cells(i, 12) = Vert) And (.Cells(i, 15) = "Open") Then
.Cells(x, 4) = .Cells(i, 13).Value
x = x + 1
End If
Next i
.Range("D3", .Range("D" & .Rows.Count).End(xlUp)).Sort _
Key1:=.Range("D3"), Header:=xlYes
If x = 4 Then .Range("D4").Value = "No Assignments Found"
End With
Call EndOfListAssignments
Application.EnableEvents = True
End Sub
See if these changes work