Both plugins Utilizes the same Right click that is correct, but i think the culprit is Objectstacks because PlaceryExtended do not allow based on your current config to even validate a vendingmachine as a valid placement and if Placery says place isnt valid then the custom code to place the box down will not even run so im pretty sure the issue lies with ObjectStacks
I dont own Objectstacks but can you try on your server maybe Remove ObjectStacks permissions from yourself and then with Placery see if you can right click place a box on a vending machine? if you cant then try Remove Placery Perms from yourself and add the ObjectStacks perms to yourself and see if its getting placed or not?
if it dosnt get placed try have PlaceryExtended and ObjectStacks Perm Enabled on yourself and then try to place a box on the Vending, if this now happends then it sure is some kind of Conflict between theese 2