Namespace Manager or XsltContext needed. This query has a prefix, variable, or user-defined function

You have to add xsl namespace to XmlNamespaceManager:

var document = new XmlDocument();
document.Load(...);
var nsmgr = new XmlNamespaceManager(document.NameTable);
nsmgr.AddNamespace("xsl", "http://www.w3.org/1999/XSL/Transform");

var nl = document.SelectNodes("//xsl:import/@href", nsmgr);

The document can be traversed by GetElementsByTagName and it doesn't necessarily need using XmlNamespaceManager:

var nodes = document.GetElementsByTagName("xsl:import");
var href =  nodes[0].Attributes["href"];

Fiddle

var document = new XmlDocument();
document.LoadXml(rawData);

var nsmgr = new XmlNamespaceManager(document.NameTable);
nsmgr.AddNamespace("cbc", "urn:xxx"); //for example
nsmgr.AddNamespace("cac", "urn:yyy");            

XmlElement xmlElem = document.DocumentElement;
var node = xmlElem.SelectSingleNode("cac:AccountingSupplierParty/cac:Party/cac:PartyIdentification/cbc:ID[@schemeID='VKN']/text()", nsmgr);
var nodeText = node.InnerText;

All namespaces that will be used in XML should be added.
Then you can access the values of the relevant nodes using xpath.