The following returns a system error of System.Xml.XPath.XPathException.

Dim Nodes As XmlNodeList
Nodes = doc.SelectNodes("soap:Body")

How is the XPath string formed where there is a prefix and a local name for
the node?

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You need to use an XmlNamespaceManager, bind that prefix to a namespace
URI and pass the namespace manager as the second argument to SelectNodes.
VB pseudo code
Dim NamespaceManager as System.Xml.XmlNamespaceManager
NamespaceManager = New System.Xml.XmlNamespaceManager(doc.NameTable)
NamespaceManager.AddNamespace("soap", "putURIHere")
doc.SelectNodes("soap:Body", NamespaceManager)
Which URI you need depends on the input XML of course.

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Martin Honnen --- MVP XML