Took a look at a Vanguard trigger. The safety a plunger that is operated by the safety lever via two different sizes holes in the body of the lever. The larger hole lets the plunger come further out and clear the trigger. Like I said, not the best trigger out there. The following is only a theory, if the safety lever is to held in position on the side of the housing, it could move without pushing the plunger in to to block the trigger. There are three small e-rings that hold the top and bottom of the safety lever in position. If any of them are missing, the safety could be out of position causing the problem.