Nice card (as NoSoup4you already noted), but why isn't it a Trap (vs Condition) or an Event (vs Attachment)? See examples below. The flavor text seems to clearly show that the rangers are lying in wait (to keep the Shire safe), and the mechanic of LOTR LCG for the rangers dealing with enemies is Traps. I don't see how Secret Vigil became a Condition Attachment.
Trap: Play Secret Vigil into the staging area unattached. If unattached, attach Secret Vigil to the next eligible enemy that enters the staging area. Limit 1 per enemy. (Same effect in other respects. Compare to Ranger Spikes.)
Event: Choose an enemy in play. That enemy gets -1 this round. When that enemy is destroyed this round, reduce each player's threat by that enemy's printed .