score:5
The Vueling Conditions of Carriage tells us, in section 8, under “Hand luggage”:
At the boarding gate, any hand luggage that does not comply with the Carrier's conditions of carriage, i.e.:
a) it has not been checked in at the check-in desk, in the cases allowed (mentioned above);
b) you have not added the service or selected the fare that includes hand luggage on board;
c) it exceeds the stipulated amount, weight or size;
will be removed and placed in the hold, at an additional cost.
(Emphasis mine).
“Additional cost” links to this page, which states:
Checked bag - up to 10 kg and max. 55x40x20 cm (cabin bag): Between 60.00 and 75.00 per passenger at boarding gate
A checked case of up to 25 kg: From 110.00 per bag at the boarding gate
The condition for the checked luggage you paid for in advance was to drop it at check-in/bag drop, where there is an optimised process to handle it and get it to the plane. At the gate it needs a lot more manual processing and handling (for which the ground service handler used by Vueling certainly charge them a hefty fee).
Depending on its size, it could have been considered “large” cabin luggage (60 euros), but then put in the hold anyway due to lack of space, or as checked luggage (110 euros).