Your question is very vague. If you need to create an XML file, this functionality is very well documented.
create XML file vb.net - Google Search[
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Simply add a web method if you need one to your webservice to do this. If that is not a possibility, create a new class that generates log files with a method that takes the parameters you need to populate the log file. Inside that method, create the log file and then utilize that method in your code wherever you need it.
There isn't much we can help you with here as we don't have access to your computer, your code, nor do we read minds or know anything about your project. If you get working on your issue and get stuck somewhere, feel free to come back with a clear explanation of your issue and some sample code so we can try and replicate your issue. If you are able to do both of those things, I am sure you'll get more meaningful assistance.