You can tidy up the code a bit more. Note the differences with this version.
Code:
Sub FiRe()
Dim FndList() As String, RepList() As String, i As Long
Application.ScreenUpdating = False
FndList = Split("257,275,299,333,363", ",")
RepList = Split("A,E,I,O,U", ",")
With ActiveDocument.Range.Find
.Wrap = wdFindContinue
For i = 0 To UBound(FndList)
.Text = ChrW(FndList(i))
.Replacement.Text = RepList(i)
.Execute Replace:=wdReplaceAll
Next
End With
Application.ScreenUpdating = True
End Sub