Page 51 of the rules lists Vestment & Surplice as prayer supplies for Fury of the Lords of Life and Death and specifies that it is purchased ‘as finery’.
Q1: Does it take up slots as per finery (3 worn; 4 packed) or per supplies (1 slot)
Q2: As expensive as it is, is it really used up after each usage, per supply rules, or is it speclal?
Q3: Given that finery takes 3 spaces worn, that means you can’t wear it and also wear a satchel or pack. Why would anyone wear finery into a dungeonn?
What you’d likely do is pack it in your gear, then take it out and put it on when it’s time to start slinging around holy stuff, like performing an Abjuration. (So, you’d set the pack down and have the others guard it while you stepped forward to get your cleric-ing on.) This is totally how it works in real life, btw. A Catholic priest coming for an exorcism will show up in their “clerics” (the black clothes with the Roman collar), and then pull out a stole (symbol of their authority) and other vestments before heading into the room to perform the ceremony.
Let me refer you to a film by William Friedkin called “The Exorcist.”