If you don't need to pass any query parameters, you can just omit that argument to QueryXml. Like this:
x=swis.service.QueryXml("SELECT TOP 1 Uri FROM Orion.Nodes WHERE Vendor='Cisco'")
If you do want to use query parameters, you can provide them in a dictionary like this:
d=swis.factory.create('ns4:dictionary')
i=swis.factory.create('ns4:item')
i.key="Vendor"
i.type="System.String"
i.value="Cisco"
d.item.append( i )
x=swis.service.QueryXml("SELECT TOP 1 Uri FROM Orion.Nodes WHERE Vendor=@Vendor", d)
