I think confirmation dialogs are a bad idea. The one for casting reopen votes is completely unnecessary on top of that.
The user needs to move the mouse to click on the OK button to cast a reopen vote. This is completely unnecessary. Of course, if the dialog is removed, second thoughts about reopening can't be acted upon. The solution is to allow users to rescind reopen votes.
In short, my suggestion is:
Remove the reopen vote confirm dialog and add the ability to rescind reopen votes.