Hilarious. Just because you can't tell the difference between a muscle, used for movement, and a nerve, used for sensation, don't expect anyone else to be impressed by such appalling ignorance.
To be fair, the plantaris is linked to proprieception because of associated receptors. It's an interesting line of reasoning though, just try not to overcook it.
There are a variety of different ways for nature to deal with a feature that no longer serves its 'original purpose', one of these is to stop bothering to develop it another is to develop it and then kill it with apoptosis, and yet another is to co-opt it to another use.
Whether or not the proprieceptive qualities associated with the plantaris muscle will prove to be ultimately 'better' than not bothering to develop the muscle at all only natural selection can tell.
While a vestigial organ has generally lost all of, or most of its original functions, it may retain or have recently developed some other functions. A vestigial organ today might be exapted for some other use in the future.
As for debate tactics, it is genuinely difficult to find a feature that has absolutely no function, no matter how obscure or redundant. Creationists will tend to argue tooth and claw trying to show vestigial features have uses and then after a long drawn out debate declare that loss of function doesn't support evolution anyway despite the fact that vestiges might originally have been raised in a different context. It's a nice bit of misdirection but remember that this topic is about homologies not vestiges with a view to discussing tautologies and patterns.
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