Optimize this Code (Vlookup-like code)

Question

I have 2 files. The first file, which will already be open when the user runs the macro has 5 worksheets. Each worksheet contains an "Order-Item" column in a different location. An example worksheet would look something like this

-Date Time Order-item Order-Quanity 
-1020 9:30 item533333 (blank)
-1020 7:30 item733333 (blank)
-1020 2:30 item333332 (blank)
-1020 6:30 item121242 (blank)

After running the macro, the user will select a file to open that looks like this:

-Order-item Order-Quantity
-item121242 183
-item333332 515
-item533333 27
-item333332 761

The macro then goes through every worksheet from the original file. On each worksheet it finds where the order-item column is located then goes through each item on the column. It searches the user-selected file for the order-item (usually column A) and looks up the quantity(always adjacent to order-item column, in this case column B)

After running the original worksheet should look like this:

-Date Time Order-item Order-Quanity
-1020 9:30 item533333 27
-1020 7:30 item733333 515 
-1020 2:30 item333332 761
-1020 6:30 item121242 183

I have created a macro that does this but as both files are rather large(the original file has about 10,000 rows and the user-opened file has upto 50,000 rows) my macro takes some time to execute. I realize I could simply do a Vlookup,filldown, then paste values and it would be much quicker; however this is part of a larger automation macro and this isn't feasible. Is there any improvements anyone could suggest to make my code run more efficent or quicker? If so let me know. Thanks!

Public Sub OpenFile()

Dim FilePath As Variant
Dim FileName As String
Dim CurrentWorkbook As String
Dim thisWB As Workbook
Dim openWB As Workbook
Dim sh As Worksheet
Dim lastRow As Long
Dim myRange As Range
Dim FoundCell As Range
Dim counter1 As Long
Dim counter2 As Long
Dim orderColumn As Long

Set thisWB = Application.ActiveWorkbook
CurrentWorkbook = Application.ActiveWorkbook.Name
FilePath = Application.GetOpenFilename(FileFilter:= _
            "Excel Workbook Files(*.xl*),*.xl*", MultiSelect:=False, Title:="Select File")
If Not FilePath = False Then
    FileName = FilePath
    Set openWB = Application.Workbooks.Open(FileName)
    FileName = Mid(FileName, InStrRev(FileName, "\") + 1, Len(FileName)) 'extracts filename from path+filename
Else
    MsgBox ("File not selected or selected file not valid")
    Exit Sub
End If
Application.Workbooks(FileName).Activate
'--------------------------------------------------------------------------------------------------
'--------------gets table range from input box.  Defailt is Row A,B--------------------------------
'--------------------------------------------------------------------------------------------------
Set myRange = Application.InputBox( _
    "Select Table Range.  First Column should be Order-item, Second Column should be Order Grade", _
    "Select Range", "$A:$B", , , , , 8)
On Error GoTo 0
'for every worksheet in currentworkbook, find how many rows there are.and find location of _
order-item. then go through each row in the order-item column and compare to column A(order-item) _
on the user selected workbook.  if match is found, place column B into order-item column+1
Application.ScreenUpdating = False
For Each sh In thisWB.Worksheets
    lastRow = LastRowUsed(sh)
    'Find Order Column
    Set FoundCell = sh.Rows(1).Find(what:="Order-Item", LookIn:=xlValues, _
    LookAt:=xlWhole, SearchOrder:=xlByRows, SearchDirection:=xlNext, _
    MatchCase:=False, SearchFormat:=False)
    If Not FoundCell Is Nothing Then
        orderColumn = FoundCell.Column
    Else
        MsgBox ("Couldn't find ""Order-Item"" in Header, exiting macro")
        Exit Sub
    End If

    For counter1 = lastRow To 1 Step -1
        For counter2 = myRange.Rows.Count To 1 Step -1
        If sh.Cells(counter1, orderColumn) = myRange.Cells(counter2, 1).Value Then
            sh.Cells(counter1, orderColumn + 1) = myRange.Cells(counter2, 2)
            Exit For
        End If
        Next
    Next
Next
Application.ScreenUpdating = True
End Sub

Answer 1

EDIT: updated to handle duplicate Id's.

Sub Tester()
    UpdateFromSelection Workbooks("Book3").Sheets("Sheet1").Range("A1:B21")
End Sub

Sub UpdateFromSelection(myRange As Range)
    Dim d, rw As Range, tmp, c As Range, arr, i

    Set d = GetItemMap()

    If d Is Nothing Then Exit Sub
    Debug.Print d.Count
    If d.Count = 0 Then
        MsgBox "nothing found!"
        Exit Sub
    End If

    For Each rw In myRange.Rows
        tmp = rw.Cells(1).Value
        If Len(tmp) > 0 Then
        If d.exists(tmp) Then
            arr = d(tmp)
            For i = LBound(arr) To UBound(arr)
                arr(i).Value = rw.Cells(2).Value
            Next i
        End If
        End If
    Next rw

End Sub

Function GetItemMap() As Object
Dim dict As Object, ws As Worksheet
Dim f As Range, lastRow As Long, tmp, arr, ub As Long

    Set dict = CreateObject("scripting.dictionary")
    For Each ws In ThisWorkbook.Worksheets
        Set f = ws.Rows(1).Find(what:="Order-Item", LookIn:=xlValues, _
                                LookAt:=xlWhole)
        If Not f Is Nothing Then
            Set f = f.Offset(1, 0)
            lastRow = ws.Cells(Rows.Count, f.Column).End(xlUp).Row
            Do While f.Row <= lastRow
                tmp = Trim(f.Value)
                If Len(tmp) > 0 Then
                    If Not dict.exists(tmp) Then
                        dict.Add tmp, Array(f.Offset(0, 1))
                    Else
                        'can same item# exist > once?
                        arr = dict(tmp)
                        ub = UBound(arr) + 1
                        ReDim Preserve arr(0 To ub)
                        Set arr(ub) = f.Offset(0, 1)
                        dict(tmp) = arr
                    End If
                End If
                Set f = f.Offset(1, 0)
            Loop
        Else
            MsgBox ("Couldn't find 'Order-Item' in Header!")
            Exit Function
        End If
    Next ws

    Set GetItemMap = dict
End Function

Answer 2

Why don't you make your VBA use Application.worksheetFunction.VLOOKUP ?