In §90 of Morphology of Biblical Greek, Mounce says nominative & vocative participles always take the same form. He cites Smyth's grammar §302. There, Smyth simply makes the same statement with no further explanation. I had a few questions about that.
1) Are they simply wrong?
2) The exceptions to this are rare in the Bible. Were they just going for simplicity?
3) In a Bibleworks search of the NT, out of 6 vocative participles, I see only one exception: κεκονιαμένε in Acts 23:3. Is it possible that Mounce is right (in a sense), and this exception is to a participle whose root was practially never used as a verb, and as far as they were concerned, κεκοινιαμέν* was always an adjective? (By the way, I would imagine this sort of process is not a terribly rare occurrence. Is there a label for it?)
4) There are more "exceptions" in the LXX. In fact, it looks like the rule is actually backwards. Out of 7 vocative participles, only 3 of them followed Mounce's "rule." What are the chances that vocative participles usually had distinct forms in 200 BC, but became very rare by the first century? Can anyone attest to this in Koine Greek outside of the Bible?