I use this simplified version of the transformation and this avoid the file to be locked:

loXslCompiledTransform.Transform(oApp.cHttpFat + "Temp\" + lcFile + ".xml", tcFile)

However, I am wondering what was the difference with this command:

loXslCompiledTransform.Transform(New XPath.XPathDocument(oApp.cHttpFat + "Temp\" + lcFile + ".xml"), Nothing, loFileStream)