Normally, the vocative form is the same as the nominative form, except for second declension nouns ending in -us. Unless there are Latin forms of these names that end in -us, I don't think there will be a vocative form.
You're absolutely correct about that. But my question is really about post-Classical usage or neo-Latin usage. As academic Latinists (say, at Wittenberg or the Sorbonne) eventually discovered or anthologized knowledge about Scandinavian deities or historical characters, and perhaps needed to Latinize invocations or addresses to deities, did they create vocative forms for these names ending in -or? I think probably the right answer is the one given earlier by Cagey, about the nominative and vocative form being the same unless the proper names were treated as second declension. My hesitation comes from the fact that I think I've since Thor Latinized as second declension Thorus--though I can't now recall where.
Once we leave the classical world, isn't it all up for grabs, so to speak?
The Church wouldn't even have written the names of pagan gods, let alone invoke them.
Medieval and Renaissance scholars would probably have tried to latinise the names with conventional nominatives first. Thorus, Brorus and Ragnorus seem to trip easily off the tongue, resulting in Thore Brore and Ragnore as the vocatives.
The novi literati of the Age of Enlightenment would have used whatever endings they thought might impress their less Latin-learned contemporaries. The conversion of Wolfgang Gottlieb Mozart's middle name into the dog-Latin Amadeus comes to mind.
I hope you don't think I'm being flippant with this, Cicerone, but I suspect that there simply is no authoritative post-Classical version of what you are looking for. There again, somebody much more knowledgeable than I may tell you soon exactly what you want to know and show me up for the dilettante that I am.
Well said, and I agree with everything you've written. I was just asking if anyone had ever actually seen any of the "up-for-grabs" forms which you and I are both hypothesizing about. I can easily imagine a scholar having concocted Thore, Brore, or Ragnore--but I haven't seen them anywhere.