I’m guessing they might use a different version of the datepicker script which is enqueued by both plugins.
On the compatibility options panel, please try turning off these options: https://i.imgur.com/zAHJu0t.png
If visual composer loads these scripts anyway, then it should be redundant, and most likely solve the problem. Don’t forget to clear every layer of cache after the change, and reload the main page by hitting CTRL + SHIFT + R to clear the local cache too.
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