I have met once a shapelifter carrying vorpal blade on the castle level. I hint that any payer monster has a change of carrying ot of not created and that the shapelifter that would be created as a player monster also has a chance (the same ?) to carry vorpal blade. Can anyone confirm wether a shapelifter is created with the inventory of the monster he is created as ?
Artifact weapons can just be created at random--I found Frost Brand lying around on the Tourist Quest in a recent game. So it is possible that your shapeshifter found it and picked it up when it shifted into an appropriate form.