Is there in English language an expression/idiom that reflects the idea that we make something vulnerable to be attacked because we want to trap the attacker ?
You may also find the word "bait" useful, as in "to bait someone" or "baiting someone", although recent usage has changed it somewhat to mean "to provoke someone into reacting (often excessively and to their own disadvantage)".
"Feigned retreat" may be useful in some contexts, but this connotes that open conflict has already begun.
"It was a setup" (or set-up?) is a phrase often heard in this case, e.g. in police shows where the police set up an artificial situation to try to catch the bad guys in the act of doing something wrong.
The "Honey Pot" suggestion is good as well. I'd say a Honey Pot is a particular kind of "setup" where the bad guys are lured by something desirable. (e.g. an expensive item that the police want the bad guys to steal, or an attractive computer that hackers want to target).
"Set-up" is a bit more general in that it could involve some other way of coercing the wrong-doers into their wrong-doing, besides desire.