Q: For masses specific to orders like the Knights of Columbus or Knights of St. John, are members of the clergy who are members of the order allowed to wear their collar decorations over their vestments, i.e. chasuble, dalmatic, alb etc.?
A: I don’t know of anything directly addressing this.
Nonetheless, the GIRM considers sacred vestments in 336, and states this: “Before the alb is put on, should this not completely cover the ordinary clothing at the neck, an amice should be used.”
To me, that implies that the people should see vestments—not decorations—when they look at the ministers.