According to Allen and Greenough’s New Latin Granmar (page 23): “The vocative singular of deus does not occur in classic Latin, but is said to have been dee; deus (like the nominative) occurs in the Vulgate.”
“Said to have been dee”…by whom? The book doesn’t appear to say.
Does anyone know?
The Oxford Latin Dictionary—much the best Latin dictionary—makes no mention of vocative dee. It’s evidently a made-up form, copying the regular 2nd-decl. vocative ending. And of course the plural is much more common.
Old textkit thread, with quotations:
Apparently Tertullian used “dee” at least once, but it may have been mockingly dismissive: Adversus Marcionem 1.29:
I saw a 16th century grammar that discussed vocative Dee, saying “apud Prudentium”, which led me to the discovery that Prudentius used Dee alternatively with Deus.