Vetoes can't be overturned like that. And every single EU country has one over an army.
BUT... Imagine for a moment that your fear are right and a nameless EU committee of bureaucrats proposes an army and overturns our national veto. What would happen next? There would be riots in the streets! I'm pro-European with a passport from another EU country as well as UK, and I would be out protesting too! Our government - left or right - wouldn't accept it (because of the civil unrest AND because they would never be elected again if they did) and would whisk us out of the EU immediately.
There are reasons - mainly ideological - for which I can understand people voting leave, though I disagree with their ideology personally. But the fear of an EU army is not a good reason for anyone to vote leave.