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I don't recall seeing that exact phrase in the texts that I've read but the analogy of the vows being used as an armor symbolizes the protection they provide to oneself and others by observing them. You can see this theme in the Self-protected Discourse (Atta-rakkhita Sutta) for example:
At Savatthi. As he was sitting to one side, King Pasenadi Kosala said to the Blessed One: "Just now, lord, while I was alone in seclusion, this train of thought arose in my awareness: 'Who have themselves protected, and who leave themselves unprotected?' Then it occurred to me: 'Those who engage in bodily misconduct, verbal misconduct, & mental misconduct leave themselves unprotected. Even though a squadron of elephant troops might protect them, a squadron of cavalry troops, a squadron of chariot troops, a squadron of infantry troops might protect them, still they leave themselves unprotected. Why is that? Because that's an external protection, not an internal one. Therefore they leave themselves unprotected. But those who engage in good bodily conduct, good verbal conduct, & good mental conduct have themselves protected. Even though neither a squadron of elephant troops, a squadron of cavalry troops, a squadron of chariot troops, nor a squadron of infantry troops might protect them, still they have themselves protected. Why is that? Because that's an internal protection, not an external one. Therefore they have themselves protected.'"
"That's the way it is, great king! That's the way it is! Those who engage in bodily misconduct, verbal misconduct, & mental misconduct leave themselves unprotected. Even though a squadron of elephant troops might protect them, a squadron of cavalry troops, a squadron of chariot troops, a squadron of infantry troops might protect them, still they leave themselves unprotected. Why is that? Because that's an external protection, not an internal one. Therefore they leave themselves unprotected. But those who engage in good bodily conduct, good verbal conduct, & good mental conduct have themselves protected. Even though neither a squadron of elephant troops, a squadron of cavalry troops, a squadron of chariot troops, nor a squadron of infantry troops might protect them, still they have themselves protected. Why is that? Because that's an internal protection, not an external one. Therefore they have themselves protected."
There are others discourses that touch on this. I particularly have one in mind that relates very closely to this theme. I'll reference when I find it.