como te decia en el Facebook una solucion es utilizar la funcion Shellandwait, esta funcion lo que hace es hacer la llamada y hasta que no termina el winrar, queda en el loop
(hay podes mostrar un progress indefinido)
ahora bien como dice cobein, si llegara a haber un error puede que no te enteres y quedarte si backup
Option Explicit
Private Declare Function OpenProcess Lib "kernel32" (ByVal dwDesiredAccess As Long, ByVal bInheritHandle As Long, ByVal dwProcessId As Long) As Long
Private Declare Function WaitForSingleObject Lib "kernel32" (ByVal hHandle As Long, ByVal dwMilliseconds As Long) As Long
Private Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long
Private Sub ShellAndWait(ByVal program_name As String, ByVal window_style As VbAppWinStyle)
Dim process_id As Long
Dim process_handle As Long
Const SYNCHRONIZE As Long = &H100000
Const INFINITE As Long = &HFFFFFFFF
' Start the program.
On Error GoTo ShellError
process_id = Shell(program_name, window_style)
On Error GoTo 0
DoEvents
' Wait for the program to finish.
' Get the process handle.
process_handle = OpenProcess(SYNCHRONIZE, 0, process_id)
If process_handle <> 0 Then
WaitForSingleObject process_handle, INFINITE
CloseHandle process_handle
End If
Exit Sub
ShellError:
End Sub