How did I get the idea that a sconce was a drinking vessel of some kind used for toasts?
Because ...
from a website with information on Oxford University:
During the meal, a student may be 'sconced' by another student over some point of honour. The challenged must stand on the bench and drink a yard of ale in one go. Having succeeded, he may immediately return the 'sconce' if he wishes. These days, rugby and rowing dinners are more often the excuse for multiple sconcing!